We don’t have any plans to translate the C++ or Blueprint API reference documentation to any other languages. The main reason is that they are currently completely auto generated.
The C++ API docs pull straight from the source code which is obviously only available in English. We’re also talking about 100,000+ pages here which would take someone a lifetime to translate and keep updated with each release.
It might theoretically be possible to build translated versions of the Blueprint API docs. I would have to look into whether the text for those gets translated with the rest of the editor text. However, as I said previously this is not something we have any plans to do at the moment.