I think this:

    if ( $trans !== null || $enabled )
    {
            if ( $trans === null )
            {
                $trans = $source;
            }
            $translation = self::insertArguments( $trans, $arguments );

            if ( $enabled )
            {
                 //...
            }
    }

Will be more readable like this:

    if( $trans !== null ) {
        $translation = self::insertArguments( $trans, $arguments );
    }

    if( $enabled ) {
        if($trans === null) {
            $trans = $source;
            $translation = self::insertArguments( $trans , $arguments );
        }

        //....
    }

But it's just a suggestion (since it may be more readable only to me :) ).