I have accounts on:
I have studied Japanese and German, but I am by no means fluent in either. The bulk of my Wikimedia contributions have been to the English Wiktionary, and most of my contributions there have been with the Japanese language entries and policy-making. Often as part of my studies, I will add vocabulary that I am learning. When I add the vocabulary, I try to determine from friends and online searches what usage distinctions exist and how best to express them. My main goal with the Wiktionary is to turn it into a useful Japanese-language reference—at once a English->Japanese dictionary, a Japanese->English dictionary (incl. romaji dictionary and furigana dictionary), a Japanese character dictionary, a phrase and proverb dictionary, and a usage guide. It has a long way to go with regards to any of these, but I am trying to increase it steadily, and hammer out a framework as I do so.
Off-wiki, you can find me at http://kurutta.net.