I have so far failed to learn much Italian, just a few useful phrases like chi schifo! (“how disgusting”). Despite having been in Italy for 5 months, I still confuse German with Italian, e.g. I plan how to tell someone “I live here” if they ask what I’m doing in this area, and I come out with “Io wohne qui”. Similarly with numbers, it takes real effort not to say, e.g. fünf instead of cinque. I notice that it happens mostly with high-frequency words like live, work, drink, want, numbers, prepositions, articles. With words like e.g. responsibility or humidity or mosquito, I have no problem.

If one generalises from my case, this suggests that when a language changes, it will be the less-frequent words which undergo mutation; the oft-repeated, core vocabulary will most likely persist.