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My first urge is to say Japanese, but thinking about it further, I may have more to gain if I learn "Classical Chinese" as the late Ming imperial scholars know it. It not only helps me with learning Japanese, but also Mandarin Chinese. It may also help me with some languages such as Korean and Vietnamese—but not to quite the same extent as Japanese and Mandarin Chinese though.

My other two choices are Latin, Sanskrit, and Classical Arabic, for similar reasons.

But Classical Chinese is it for me. Maybe I can do Kanbun within my lifetime through it and Japanese.