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If I could learn one language I don't know why I would pick... I'll have to think about it. A background in Latin is particularly good for the Romantic languages (French, Spanish, Italian) but has pretty limited practical use.
Learning Greek letters is really useful, just because they get so much use in maths, physic and science generally.
Frankly though Hell, your English is so good I think you should be proud of your linguistic achievements already.