I work in a full bilingual English/Spanish environment where I must access and generate documentation in both of the languages.
I have experience as proofreader for Project Gutenberg.
I can translate English to/from Spanish and also in a more limited level of experience, Brazilian Portuguese.
No Japanese reading/translating abilities, though (really sorry about that) so I use English translations as originals.
All of my translations are free to use. The only condition I require is to credit me and the original JP->EN translator. After all, she/he did most of the work :)