The thing also is, the more proficient you are in more languages, the easier grasping other, related ones becomes.I'm a native English and German speaker and I speak:-
French (professional fluency) , Modern Greek and Italian (conversational),
Spanish (basic active and intermediate passive knowledge)
And some school Latin...
Like being a native English and German speaker, getting the gist of and especially reading Dutch/Flemish is no big deal. Yiddish is 80% medieval German, and especially if one if familiar with the Berlin region dialect, basic understanding comes easy.
Same applies to the Romance languages- if you know Italian, reading Romanian or Catalan is not too difficult. Or the combination of Italian and Spanish also makes reading Portuguese possible (...understanding in that case actually is easier the other way around because of the accent and pronounciation).
Ironically, knowing Modern Greek even gives you basic vocab knowledge in Turkish because the long Ottoman rule in Greece with Greeks running a significant portion of trade and administration in Constantinople/Istanbul has lead to a pretty significant pool of loanwords in both languages.