English and other translation within single paper
I would think it is probably a mistake to do this. While it is possibly convenient and makes the paper more widely available, it will cause some problems for reviewers. Most of the available reviewers will speak one language or the other only and will either have to trust that the translation is adequate or find some other means. At best this will slow you down. Math especially may be a difficult field in which to do this as machine translation is likely not as good as for (some) other things. 

The big issue is, do the two halves, really say the same thing. That can be a bit subtle. It isn't impossible to deal with it, but it would be a strike against the paper in the minds of editors, I think. 

Probably not a good idea. Publish in the language you are most familiar with and provide a translation separately. 

Carefully mark the translation as a translation and reference the original version within it.