I’ve checked this http://docs.dhtmlx.com/doku.php?id=dhtmlxscheduler:localization
and I have modified sources/locale.js but plugin still appears in English…
what am I doing wrong?
Save updated file as
event-calendar-scheduler\codebase\sources\locale_en.js
no success yet… 
The logic is next
Scheduler checks if such file as
sources/localue_{your locale code}.js
exists, and if such file exists it will be loaded.
IF ther is no file with such name, it will load sources/locale_en.js
Be sure to clear cache, because old version of js file can be just cached by browser.