Theme check: Missing a text-domain

Let us look at the relevant line. I am reformatting it to easier understand what is going on:

printf(
    _n('1 comment', '%1$s comments', get_comments_number() ),
    number_format_i18n( get_comments_number() ),
    'text-domain'
);

So what you are doing is calling printf with three parameters, each one now being on a separate line now as I formatted it.

But this doesn’t really make sense, especially the third parameter text-domain doesn’t make any sense there.

What you probably want is using _n to translate using a number that you put through number_format_i18n.

So I guess this is what you’re looking for actually:

printf(
    _n( '1 comment',
        '%1$s comments',
        number_format_i18n( get_comments_number() ),
        'text-domain'
      )
);