This code:
$TALTranslator = new \PHPTAL_GetTextTranslator();
$TALTranslator->addDomain('mydomain', './locale');
$TALTranslator->useDomain('mydomain');
$TALTranslator->setLanguage('', 'en_US'); // Empty string as first argument
Will throw this exception:
Language(s) code(s) ", en_US" not supported by your system.
Looks like some sort of strange string concatenation problem as indicated by the leading comma in the list of languages in the error.