I'm German and that is bullshit. Never heard of mäusespeck, everyone just calls them marshmallows and they are labeled as marshmallows in the store
EDIT: I was made aware that the Problem seems be that im not a boomer. 30 years ago, when i wasnt alive, they seemed to be called this. In my WG there are people over 30 though and they also never heard of this (hessen)
In Dutch it's also marshmallows, but also commonly spek (bacon), spekjes (bacon pieces) or spekkies (in this case it's clear you're not talking about bacon).
I’m pretty sure alot of languages just imported marshmallow.
I come from the german speaking part of Switzerland and I don’t think I ever heard someone actually use the word “Mäusespeck” although it certainly would he understood I think. Everybody around me calls them marshmallows.
Here in Mexico (Spanish) is known as Bombón. A quick Google search says that apparently comes from the repetition of the french word 'bon'
It is worth to mention that, despite most of the countries in Latin America speak Spanish, things have different names in different countries, even if it's the same language.
According to Wikipedia, marshmallows are also known as
nube, esponjita, malvavisco, fringuel, jamón o suncho
In Norwegian marshmallow is just marshmallow, but if something has marshmallow bits or marshmallow like properties (like say the white stuff in a bag of Haribos) it's called "skum", which means foam
Never heard of that in German. It's just marshmallows with a generic German accent instead. But it's cool to learn something new. Which region says that, OP?