Why English language is sometimes "lazy", sometimes not

It really depends on how the vocabulary got to the language.

First, you have to start with the fact that English is a Germanic language with a lot of French words in it due to the Normans. A good example is that farm animal names have a Germanic origin typically while the words for meat have more French origins.

You can also combine it with the fact that English was rather varied in its vocabulary at the time. An example is eggs/eyren. Egg won out as a word, but there were likely thousands of these decisions being made as the language standardized.

Then, include that a lot of legal and religious terms came from Latin; with the practices using Latin far beyond when English became the dominant language in England. So, for these fields, just use a mangled version of the Latin root.

Finally, it depends on how a word enters the English language. Maybe it comes from another language, so we just use a version of that word. Maybe the creator of the word just makes something up. Maybe the creator just slapped a word on something that wasn't typically used; the wiki from Wikipedia comes from the name of a Hawaiian bus.

It is a strange, made-up way of naming things, but language itself is made up thing.

Because English is half a dozen languages wrapped in a trenchcoat? A lot of the law related words are from French derivations AFAIK?

Domain specific language is found across all realms of society, even firefighters have words or phrases with a specific meaning (back-burning, pumper, appliance, etc). So maybe its not that some areas are lazy, its just that you haven't been exposed to their more technical side?

Because English is half a dozen languages wrapped in a trenchcoat?

A language is not its vocabulary; that's like pretending that the critter is just its fur.

English vocabulary is from multiple sources, but that is not exactly unique or special.
Law terminology specifically can seem pretty archaic because there's a high need for terms to be stable over time. In other fields and everyday speech terms can change over time. There's contracts signed decades or even centuries ago that are still binding today. So it's practical in a sense if the words within and those used to discuss legal dealings don't change over time.
A lot of the law related words are from French derivations AFAIK?

Yeah, those legal terms have been more-or-less common to any romance language I've used.

All languages are the result of the collective brainfarts of lazy people. English is not special in this regard.

What you're noticing is two different sources of new words: making at home and borrowing it from elsewhere.

For a Germanic language like English, "making at home" often involves two things:

compounding - pick old word, add a new root, the meaning is combined. Like "firetruck" - a "truck" to deal with "fire". You can do it recursively, and talk for example about the "firetruck tire" (the space is simply an orthographic convention). Or even the "firetruck tire rubber quality".
affixation - you get some old word and add another non-root morpheme. Like "home" → "homeless" (no home) → "homelessness" (the state of not having a home).

The other source of vocabulary would be borrowings. Those words aren't analysable as the above because they're typically borrowed as a single chunk (there are some exceptions though).

Now, answering your question on "why": Norman conquest gave English a tendency to borrow words for "posh" concepts from Norman, then French. And in Europe in general there's also a tendency to borrow posh words from Latin and Greek.

Language is always evolving. A lot of “special” words are just lazy words that have fallen out of regular use over time, or have be pulled out of time and place to evoke the seeming of being old and authoritative. Sometimes "special” words or phrases are just memes used out of context, and sometimes the context is no longer relevant or it is forgotten. We have a “special” word for phases like that: Idioms. The rule for idioms is “Idioms mean what they mean”

Second point: the English language is heavily influenced by several historical processes

WARNING: I am not a linguist or historian and the following is greatly simplified, potentially to the point of falsity

The invasions of Germanic tribes: Angles & Saxons most notably, settled in what we now call England (Angle Land) and pushed the Celtic tribes west and north. Leaving mostly Germanic speaking peoples in the south and East.

The Vikings raids: another wave of Germanic speaking peoples raided and eventually settled in parts of the island, while no less violent than the earlier invasions, it did result in more intermingling of the local Germanic and the Norse Germanic languages than the previous Germanic/Celtic languages did.

The Norman Conquest: This invasion was more of a top-down invasion, where a French speaking monarchy replaced the English speaking monarchy. For a time French became the language of esteem, and state business was conducted in French, while outside the aristocracy, the common folk would use common English in their day-to-day. This is why a lot of modern legal and technical words, like litigate, defendant and plaintiff, have roots through French while rude words (“vulgar” comes from the Latin for “common”) often have Germanic roots. See: penis/vagina/intercourse vs. dick/cunt/fuck

Colonization and globalization: English speakers went out and invaded a lot of places. In addition to extracting resources, wealth and slaves from those places, they took a lot of words too, and just kinda squished them into the language where they could fit. Colonizers also forced English upon the invaded territories much like the Norman’s forced French upon England. Now you have many more English speakers in the world who are also have fusing their own languages into local dialects of English and English words into their native languages. All this gets mixed up into an era of global trade, travel and communication, and some words just get caught up in the global zeitgeist and make their way into common English usage.

Also, the Church and Romans are mixed up in there somewhere, but I have forgotten how.
- Re: #6 prior to the invasions of the Angles/Saxons England was invaded by the Romans and a number of cities sprung up in the south populated by latin-speaking peoples. When Western Rome collapsed, the state capacity needed to maintain large urban centers went with it. The Roman inhabitants didn’t leave, though, but instead mixed with the local culture and the language dispersed into the general population of England.
  
  At the same time the only thing left of Western Rome’s larger institutions was the Church, whose organization was inherited from Roman civic structures and who were the ones preserving written knowledge, which would be written in Latin.

Most often the "fancier" words are loanwords from other language. Plaintiff/Defendant are from the French "pleintif/defendant", litigation is from Latin. Firefighter, firetruck, and other compound words were created relatively recently compared to the others. Firefighters, firetrucks and newspapers mostly didn't exist until after English mixed with other languages.

Also, the number of loanwords in English is completely absurd. Some other languages resisted borrowing/stealing special terminology from other languages by coming to with their own clever new words.

For example, entrepreneur is a clear loan from French where a salesman is a simple and clear description of a man who sells something. If you don’t know French, you’ll have no idea what the word entrepreneur means, but if you know basic English, salesman should be crystal clear to you.

Many other languages developed lots of these types of clear words in order to make communication easier and less elitist. English is completely wild and there’s no central authority that could reasonably give any recommendations that anyone would listen. This sorts of uncontrolled wild growth and stealing has been going on for centuries, and now we’ve ended up with a complete train wreck of a language.

And that’s just the tip of the iceberg! Wait until you hear about the history behind how spelling and pronunciation became the disaster we have today.
A lot of them haven't changed much in the last few centuries either. Law is a pretty arcane thing. I also suspect they don't want most people to understand it too well. If the legal system is a confusing, overcomplicated, bureaucratic nightmare, then the lawyers will always have job security and charge stupidly high rates for their work.

In general, specialized disciplines (like law, medicine, science, etc.) tend to also use specialized words. I don’t think English is unique in that regard, other languages do this as well.

Many of those words aren't actually limited to those use cases, but they are used there because they have very specific meanings. A plaintiff, for example, is the person lodging a complaint. Doesn't have to be a legal complaint, but in legal terms it makes it very clear who we're talking about.

Others are just expressions. A snap election is just when you call an election earlier than the scheduled one. It's essentially a nickname for something more complicated. Same goes with casting a ballot. It just means to toss your vote in for the count. It's just the expression that stuck because it sounds fancier than just saying "voting."

A plaintiff, for example, is the person lodging a complaint. Doesn't have to be a legal complaint, but in legal terms it makes it very clear who we're talking about.

I think the OP was wondering why that person wouldn't just be called a complaintlodger. Like with firefighter.
- A quick "rule" is to see how old the word/concept is. "plaintiff" would have existed almost as long as the English legal system came into being, or probably even older to the court of Assizes pre-12th century.
  
  Whereas firefighter as a profession might have only become a word after the establishment of fire departments by insurance companies, which I think might have been a 19th century development.
- Or complainer....

A lot of the top set come from latin and/or french (sometimes borrowed from one into the other first). Lots of words around the legal system, government, nobility, etc. come from those roots. Many from the Norman conquest but some earlier. Some even got borrowed in twice (not french but both shirt and skirt are borrowings of the same word at different times).

A lot of diplomacy was also french be cause that was the language for diplomacy for a long time. For some sciences, it was German.

A lot of the more working-class, I guess, and later words follow the old Germanic patterns (the base of a lot of old English coming from Anglo-Saxon and, to a lesser degree, old Norse)

This is exactly it, and it has to do with the Norman Conquest. After 1066, French was the language of the ~~hurling~~ ruling class and English was for commoners. As such, a lot of French words got borrowed into English, and they usually carried a higher status. Cow vs beef, deer vs venison, that kind of thing.
- IIRC it was around 10,000 French words that were introduced into English after that. That’s what we learned in school, anyway.

Can someone explain why a job application called "resume", like in Play/Pause/Resume?

How is it relevant?

(I'm learning English as second language).

Edit: So we're speaking French now? What? Why? You guys butchered so many words already, can you just made up one more?

Ps: Is that also the case with the word "fiancé"? I've been wondering where the hell did that "é" came from.

Because it's actually supposed to be spelled résumé, being a word borrowed from the French
resume (cv/job application) more recently comes from french and is pronounced differently than resume (to continue). Ultimately they both come from the same latin word "resumere". https://www.etymonline.com/word/resume
As part of a job application, it's called a résumé (reh-zuh-may).

To continue playing something, it's resume (ruh-zoom).
Here is a good explanation of both words, basically play/resume comes from Latin (take up again) and CV/resume should be résumé, the past participle of resumer, to sum up, because you’re takin a summary of your experience.
"They both come from the same French word, résumer. The Latin root of that word could be translated as "to take back" (and you can see that use on occasion, as when a deposed king "resumes the throne"), but the core meaning that links resume and résumé could better be translated as "to come back to" — in one case, to come back to an activity; in the other, to go over a series of events again in brief."

But I'll tell you as a native English speaker, we almost never think about words like that having different meanings and origins. We're just really used to homographs and homophones.
The job application one should be pronounced with a long a as the second e. Despite the last e not being silent the u is still elongated. It's a recent adoption from French. Even though they are spelled the same the two words are unrelated.
- the u is still elongated
  
  Eh? Resume is /ɹəˈʒuːm/ or /ɹəˈzjuːm/. Résumé is /ˈɹɛz.(j)ʉˌmeɪ/. That's in my accent and other accents will vary in the precise vowels used. But because the accent is on the first syllable in résumé, the vowel becomes de-emphasised and, in many accents, more centralised. And that is, as far as I'm aware, nearly universal among English speakers.

many legal terms come from latin. same with science, especially medical/biology. Basically philosophy (which law branches out of), medicine, and clergy were some of the first codified degree type of things.

I think what you mean is compound words vs other words?

Wikipedia says there are lots of compound words in English.

Plaintiff is borrowed from Old French. Litigation from Latin...

I suppose it boils down to when and under what circumstances a term was needed to describe something. Sometimes there was a word from another language available. Or the whole subject came from a different culture. And sometimes they just described it with a compound of what it resembles. And how to make up terms probably also depends on what is en vogue at the time.

If everyone in a town has a house, could you say that town is homelessnessless?

could you say that town is homelessnessless?

Has. The word is legit, but it would be an adjective because of the last -less there, so:

That town has homelessnessless.

That homelessnessless town is nice.

You could convert it back into a noun, through zero derivation; for example "homeless" is an adjective too, but people can say "the homeless are hungry", as if it was a noun. But it sounds weird in this situation, I don't know why.
- So, you are saying I should ask my representatives to focus more on achieving homelessnesslessness?

I've never thought about it like that before! Thanks for giving me a new way to look at my native language lol

You would love https://historyofenglishpodcast.com/
- Thank you! That looks really interesting!

https://www.waywordradio.org/

This is fun