First of all, thank you very much for generously providing such a great site free of charge to all of us.
I just found Japanese translations of Jātaka (38, 57, 208 and 308) on the Internet.
http://www.manduuka.net/pali/reading/index.htm (down the page, you will see baka-Jātaka, Vānarinda-jātaka, Susumāra-jātaka, Javasakkuṇa-jātaka.
The site administrators say that anyone can freely link their pages. I wondered if SuttaCentral team would like to have a look to consider the pages be included in the Sutta Central.
On their website, people can choose “translation exercise” pages or “translation only” pages for each listed Jataka stories.
E.g. Jātaka 38
Exercise page 1 (by clicking ALL DISP, people can see Vocab. list, Japanese translation and comments) The page includes translations of commentaries.
It continues to page 6.
If you would like to see next page, click NEXT button in the hyperlink, or change the number to 0382 in the URL.
I cannot read Pali but am capable of handling Japanese and English. If I can help you in any way, please let me know.