Let's say I have some Japanese word like 日当たり and I want to add Russian translation, since Russian is my native. Currently I get "A word with that reading and writing already exists." error. There are a lot of words that don't have Russian translation, about 9/10 in my lists. So, is support for this functionality planned for later updates?

Yes, one of the projects we're working on right now is a new popup that will replace this page:

http://www.skritter.com/vocab/word?lang=zh&base=%E8%84%91%E7%94%B5%E6%B3%A2&vari=0

You'll click on the word in the list and see a popup which will show you all sorts of information and let you add a definition in your language if it doesn't already exist. Or set a custom definition/submit a correction.