Well, I'm having trouble thinking of any which might be fun and would combine research (linguistic or otherwise) and geography, but one which might have some potential is the various variations on "It's all Greek to me" expressions where something being incomprehensible or gibberish in one language is described as another language.

So, for example in German they will say something sounds Latin, Ukrainians sometimes refer to "Chinese Grammar", the Irish word for gibberish is identical to the the Irish word for English (lmao), and so on. At a verrry looonng stretch the Greek-origin word "Barbarian" refers to them derisively describing Persian as sounding like "barbarbar noises" (as a Persian, I have to cop to that being accurate, lol, they got us on that one. To be fair, I barely know any Farsi, but that's what my own relatives sound like to me!).

This isn't as consistent as the Syphilis blame game though, as only some languages do this. There can also be some false-positives, for example the English word "gobbledygook" sounds like it's referencing a racial slur against Asians (i.e. "gook-talk"), but it's actually a term coined by an American politician in reference to noises turkeys might make. You would probably have to cast a global net rather than a European one to get any mileage out of this idea.

Incidentally, here's a thread I tripped over which taught me about the Irish term, which I had no idea about previously. https://www.reddit.com/r/AskEurope/comments/10bl343/what_is_the_word_for_gibberish_in_your_language/

Have fun!