The question of languages is very interesting. I studied Czech at the Defense Language Institute in Monterey, CA. Here are a few things I'd like to add:
1. You are a gifted linguist or not. To get into the school you take a test which utilizes a false language. Based on your score you are accepted and then assigned a Catagory of Language. CAT IV languages are Arabic, Chinese, Korean because they are tonal, a word can be pronounced with different emphasis on a syllable and have a different meaning making these a very challenging 2d language. CAT III are the slavic languages, Czech, Polish, Russian, etc. and German. I can't remember the others but I believe the CAT II were the Romance Languages, French, Italian, Spanish.
2. The difficulty of a language contributes to how proficient you become and your natural ability.
3. Czech, Russian and Polish have some words that are similiar just like Spanish, Italian and French have similar words. If you learn one language in a Catagory its much easier to learn another in that same catagory. To a nonspeaker Czech and Russian can sound very similar but Polish is much "swishier" and distinct.
4. Once you have a second language it is much easier to pick up a third or to understand it. I can easily see how someone who spoke Polish could be asked a question in Russian and understand it but not be able to reply in Russian.
From my experience with linguists, you had people who were naturally gifted with languages and those (like me) who weren't and struggled to get proficient.
As this relates to AA and FS, I think it just depends on if they were naturally gifted with languages. I also worked at Disney World in High School and managed to pick up enough Spanish to take orders etc just by being exposed to the language and with no training. I would imagine if you were growing up in Poland at different times you were probably exposed to Russian and to German as well as the native language and could pick up quite a bit.
