How many languages can we learn at the same time?
Is there anyone who learn more than 2-3 languages in SuperMemo? Do you have any memory interference?
A good idea is to learn individual words in context, e.g. as clozed sentences. You may need more than one item for each word to succeed, but if you learn French words in French sentences, you are less likely to push in German words. If you try the "traditional" method, e.g. Polish-French, Polish-German, you could struggle, esp. when questions are identical and only a language label differentiates between the two.
I tried a few languages. I kept databases separate. I did not see much problem, but I quickly gave up all languages and now only learn English. Even with SuperMemo it takes too much time to learn many languages. Unless you really really need them.