Code Geass with language borders, part 2! Since everyone seemed to love part 1 so much.
C.C. knowing dead languages! Understanding ancient texts and translating them, a talent which no one can fathom, yet Lelouch very much admires, and not just because it can get handy at times.
Lelouch himself might be familiar with Latin. Bonus: he and C.C. choose to argue in that language to make use of the highly specific terminologies. Extra bonus: the Black Knights turn Zero's linguistic abilities into a drinking game—one shot everytime he switches languages, two shots if he switches to a language no one knew he could speak.
Multilingual Mao! C.C. teaching him a few of the languages she knows to give him something to fixate on, to help his head find relief. Memorising proverbs and poems in other languages really helps Mao, and just about any activity with C.C. makes him very happy. They'd mainly communicate in Chinese, which Mao would use as leverage against Lelouch, since it's one more special thing between him and C.C. that Lelouch can't interfere with. Of course, Lelouch shoots him with a "I wouldn't be sure about that" in perfect Chinese and Mao loses it.
Rolo being taught Japanese before being sent to Lelouch, with prior knowledge of how Lelouch speaks Japanese at home. He learns it fairly quickly, but makes a little blunder every now and then. Lelouch takes it upon himself to strengthen Rolo's Japanese, teaching him more kanji and proverbs. Countless nights in front of textbooks, and Lelouch being a sweet and patient mentor, are very special to Rolo.
Tianzi not knowing any language other than Chinese, as she has never had a chance to communicate with people outside of the Chinese Federation. Xingke, who knows a couple of languages himself, teaches her whenever he can, and promises her she will use those languages to communicate with people outside one day.
Gino deciding to learn Japanese from Suzaku. He has no particular reason to learn it and probably just does it for fun, and because he's tired of only speaking English. Anya joins in, out of boredom. Suzaku is not used to Japanese anymore, especially since he became a Knight of the Round, and at first finds indulging in the language again surprisingly saddening, as it reminds him of the times before the war and him teaching Lelouch Japanese as kids.