Is there a way to change editor interface language? I'm stuck with a half-translated version, which was selected by default, and I'd like to just use English.

You can change editor source code and set:
App.lang(EN);

Where exactly in the source code should I set it?

Probably best in InitPre()

Yes, but in what file? Sorry, I'm a bit lost in the source, and there are several files with InitPre

UID(3973754331, 1094971886, 2714907782, 3847274065) /* Titan Editor\Main */