Thanks, Helen. Now we could clearly see that the problem with TinyMCE isn't located in language packs - the problem is absent on qa.moodle.net with latest packs.
Now we should understand, why moodle.org does /lib/editor/tinymce/extra/strings.php call, while qa.moodle.net doesn't. Could it be because moodle.org code wasn't updated for some time?