As Seth says, multiple comments saying the same thing are bad. They take up screen space that should be available to comments of a less meta nature. Also, they make us look unprofessional and are probably far less likely to be taken seriously than a single comment, especially if the single comment is upvoted.
Anytime there are non-constructive comments, including comments repeating the same basic idea that other comments have already said, you should flag them for deletion.
With that said, sometimes it is acceptable to comment about how Ask Ubuntu is English-only. You may want to edit your question with links to examples of this (though I realize the comments might have been flagged and deleted already).
You're right that if someone is going to bother to comment about how the site is English-only, they may as well also edit the post with a machine-generated translation. I agree that commenting on a non-English post to tell people to use English, without providing at least a machine translation, is silly and counterproductive. (Unless it's a language for which no reasonably acceptable machine translation can easily be produced.) If we can make a non-English post English and thus adequately clear and on-topic, but don't, why should anyone take us seriously when we talk about policy?
(Of course, some non-English posts are worth closing for other reasons. For example, they may be unclear, or off-topic. But even then, editing in some translation usually helps clarify that and move things along. Anytime a non-English post cannot immediately be made clear enough to stay open, a close flag/vote is in order. If no adequately clear translation can be provided--or the OP has explicitly insisted on help in a non-English language--then in addition to voting to close, a single explanatory comment would be in order.)
In my opinion, the comments about Ask Ubuntu being an English language site that are valuable are the ones that suggest alternatives for users who prefer to use another language. Of course, which alternatives exist depends on which language someone has posted in, and the comments would ideally be posted in both English and whatever language the OP has used, which might be why there is no widely promulgated pro-forma comment for this.