I’m going to diverge slightly from the original question/requirement here. English is my mother tongue and I can easily get around in French but, (and here we deviate), by far the most useful thing I have learnt is Greek!!!

Let me explain. I learnt Ancient Greek at school and I can remember quite literally zero! The big bonus here is that I do remember the alphabet and whilst it is somewhat useful in Greece for holidays it has a lot of the same/similar characters to Cyrillic. Many of the others are the same as the Roman alphabet and that leaves one just having to know about 4/5 other characters.

We deal a lot with Russian clients at work and when I was first in Moscow a number of years ago, (not actually that many years - about 13), almost everything was in Cyrillic. Without the Greek alphabet we would have been stuck on the metro system. Station names were all in Cyrillic so getting off at the correct stop needs concentration.

Rather a large amount of Russian is the same as English I.e. address, contract and passport are all the same but in Cyrillic. OK, so for the pedants one may drop the odd second character, and c becomes k as C in Cyrillic is actually S but you get the gist.

We go to Cyprus quite a lot on holiday so the Greek alphabet helps here too but in short, (clearly the above is not a short explanation), I vote for Greek, or more accurately the Greek alphabet which can be learnt pretty well in about 10 minutes.

My 2 cents.