Pointing to that issue here https://github.com/arastta/arastta/issues/53, I want to ask how we can handle such in future.

The 'problem': some extensions are using language tags like this (note the underscore):

de_DE

Now we have in the default.php this:

$_['code']                  = 'de-DE';

Shall we now add another variable or - maybe better - a new function through the library language.php where the code is converted into the needed string, like

return str_replace( '-', '_', $data['code] )