This question might seem very broad or opinionated but I did not find a better way to put it in short.
I like to travel to the Czech Republic and often times my English and German combined help me be understood. But that's not always the case. Because of that, I contemplated learning either some Czech or some Russian.
Many people in the Czech Republic speak Russian and this language would be more helpful in all eastern Europe. While Czech would be more helpful there, it wouldn't help as much elsewhere.
I already learnt the Cyrillic alphabet some years ago (my ex-GF tried to teach me Bulgarian and I could read the menu in restaurants) so it should be easy to reacquire that knowledge.
So you could say: should I aim for specialized knowledge or broader knowledge?