It Should be possible to search content only on specific languages/translations.

Simply adding a LanguageCode Criterion to the search does not allow for this, instead filtering on content which contains that translation.
In the following example, the FullText returns results regardless of language:

Take the following example command snippet:

        $query->criterion = new Criterion\LogicalAnd(
            array(
                new Criterion\FullText( $input->getArgument( 'search_text' ) ),
                new Criterion\LanguageCode( $input->getArgument( 'language' ) ),
            )
        );
        $result = $searchService->findContent( $query );

Steps to reproduce:

1. Create an article in English ('eng-GB') with content 'test',
2. Create a translation in Portuguese ('por-PT') with content 'teste';
3. Search for the word 'test' and language 'por-PT' : article is returned in the results.
4. Search for the word 'teste' and language 'eng-GB' : article is returned in the results.

FullText (and possibly other criterions) should allow specifying LanguageCode(s)