Yes, there is the %moodledir%/lang/en_utf8-folder too.
The core language files reside here, but additional blocks or modules can have their own %moodledir%/mod/modname/lang/en_utf8/modname.php too.
Files in %/lang/en_utf8 will override %/mod/modname/lang/en_utf8
In the site admin / languages / edit language window / module selection combobox the path of the basic language file will be displayed, if it is not in the core language folder.
Custom certificate (plugin)
Where are the localizated strings?
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