I have too many contenders.

Useful languages for travel are Spanish, French, Portuguese, Arabic, maybe Bahasa

Languages relevant to my religious/cultural background (and useful for genealogy) are Hebrew, Yiddish, Russian, maybe Lithuanian (to read some records but only from the interwar period)

Languages that are not of much practical use but still interest me are Esperanto, Latin, Ancient Greek, Tok Pisin (aka pidgin), Gaelic