How can I include the language a user has selected as a field in a report ?

So that if he has chosen French, it shows French in the Language column, if English, English etc

I suppose you have a language field in your form, is that correct? You can create a condition in your form that will update a field's value depending on the language being selected.

Here's how it works:

If you don't have a "Language" field in your form, I suggest you add one. And, if you don't want your respondents to see it, you can just hide it.