"the/a". If you're a native English speaker, like me, it probably doesn't look unusual. I was listening to a lecture series on linguistics and it wasn't until then that I learned that most languages out there don't have a mandatory definite/indefinite article. In most languages, if you want to say "cat", you can say "cat". English requires you to say "a cat" or "the cat" -- the presence of an article to indicate whether the thing you're talking about is unique or not. That's an unusual feature for a language to have. It's baked into how I think, but a lot of the world just doesn't work that way.
Articles are found in many Indo-European languages, Semitic languages (only the definite article)[citation needed], and Polynesian languages; however, they are formally absent from many of the world's major languages including Chinese, Japanese, Korean, Mongolian, many Turkic languages (including Tatar, Bashkir, Tuvan and Chuvash), many Uralic languages (incl. Finnic[a] and Saami languages), Hindi-Urdu, Punjabi, Tamil, the Baltic languages, the majority of Slavic languages, the Bantu languages (incl. Swahili). In some languages that do have articles, such as some North Caucasian languages, the use of articles is optional; however, in others like English and German it is mandatory in all cases.
"data". It used to normally be the plural of datum, but within living memory has normally become a mass noun, like "water" or "air" or "love". It's not the only word to do this, but it's unusual.
"deer". It's not the only word to do this either, but it's one of a small number of words in English where the plural and singular form can be (and traditionally, needed to be) identical. Today, it looks like regular forms of these are increasingly being considered acceptable, at least in American English ("deers", "fishes", etc).
Japanese doesn't have articles or account for number with something as simple as an s (some words could take -tachi or -ra as a plural marker, but not all, and often it isn't even used when plural unless there's specific need for it). Often, we learn something is plural by other inference or a number given. My wife has a hell of a time with articles and the like when trying to speak English.
I'm also learning modern hebrew (Arabic's writing system seemed a bit much plus all the dialects vs written MSA, so that's now a later goal) and they only have definite articles so the indefinite is the default state.
Although using "data" as both singular and plural is acceptable in modern English, I once sat through an online training stating "[there can be] negative consequences if data are misused or falls into the wrong hands" which is just so cringe!