Best and only documentation is SDK commit log, period.
FOnline documentation always has been outdated, even russian one, so you can imagine how behind english one is
What you'll get from BuildEn* are basics really; good for start but sooner or later you'll reach stage "there's no way X is not implemented" - that's when you should switch to commit log. I've been there, started with english .chm, then translating russian one, and log as last step.
As
cthulchu mentioned, Google Translate is your only sane option (beside fellow russian-knowing dev, ofc), and let me tell you, it improved a lot when it comes to translating code-related stuff. There is still some gibberish around -checked random commits few months back- but it's just few words/sentences overall, you don't have to decipher half of entry with two other devs