Dear Robert, Thank you very much! I solved my problem -
1. Adding "&locale=he" to the url works.
2. In order to sense the Chrome language (with no locale parameter in the url), I did 2 code modifications:
2.1 - First modified setLocale() code in init.js to "prefer" navigator.language upon cookies. For applications it's enough, and so for Launch button in WAB, but for WAB window itself, the widget label (for in-panel widget, when I hover on it) remained in english, so I had to do 2nd modification:
2.2 - in jimu.js ConfigLoader.js in function _checkLocale() - when setting appLocale, I moved dojoConfig.locale to the start of the "or" sentence.
Now the Chrome language is sensed even if no locale parameter in url.