What would be the most common way to say guy and girl/lady in French? (Assuming you are talking about someone in their 30s or 40s and want to be polite but not too formal).
Today I was speaking in French about the guy who I do a skype French language exchange with.
In English I would refer to him as "the guy who I do a French language exchange with" however I didn't know what to say in French. Is homme too formal? I could refer to him as my friend but this wouldn't work for situations where I don't know the person well. I wasn't sure whether other words were just used for very young people.
There are different possible translations for guy in French. Those are
Mec : most familiar of the four solutions
Type : designates a rather ordinary personne. So someone you would't know mostly. (e.g. Ce type dans la rue -> This guy in the street)
Gars : this one would be the most appropriate for your case, it is familiar but not that much. Can be used with friends (e.g. Allez les gars ! -> Come on guys !)
Homme : a better translation would be man, it is the most formal of the four.
Monsieur : highly formal. (e.g. Ce monsieur assis au bar -> This gentleman sitting at the bar)
as for the feminine equivalent gal:
Nana, gonzesse, nénette : familiar words that are falling out of usage
Meuf : familiar also, this is the verlan (reverse) version of the word femme that may be one the most used by people under 30 but can only be used orally.
Fille : not familiar at all, although you would not use it in front of the person you are talking of. You could however say : Cette fille là-bas ! which would be translated by : That girl over there!
Femme : most formal alternative that would be translated by woman rather than gal
Dame : highly formal, equivalent of the english lady
Finally for both men and women, one could use :
Personne : this is the non-genred version of man and woman. This is formal but less than dame or monsieur
As Laure suggested you can also use the words celui ou celle although you wouldn't translate this to guy or gal but rather to the one (e.g. Celui avec qui je vais au cinéma -> The one with which I'm going to the movies)