I completely forgot about Latin for some reason! I guess I was stuck in a living-languages track. I took GCSE Latin, and it remains useful in a grammatical and etymological substructure kind of way. (German and Latin were the two things that really made me aware of the grammar of languages; French didn't really do much for that because it's close enough to English in a number of important ways that a lot of that was just skated over in class.)

Even the minimal Irish I've learned is enough to let me puzzle out Scottish Gaelic conversations on Twitter without too much work and occasionally not even a dictionary (hi, khalinche!). I'd love to acquire enough spoken facility to find out how mutually-comprehensible they are in that mode as well. Welsh remains a bit of a mystery apart from spotting the odd distant cognate across the P/Q divide, though.