The standard term for a 'programming language' (from y Termiadur - http://www.termiaduraddysg.org/?lang=en) is iaith rhaglennu - 'language of programming' if you wanted to be literal.
In this case, the verb-noun rhaglennu is not acting as an adjective, so it does not mutate following the feminine noun iaith. (In some other cases it might act as an adjective and it would mutate, ...)
Yes :) Unfortunately due to the way the incubator works, we'd have had to teach "rhaglennu" and "raglennu" if we wanted it totally correct. The mutations shouldn't be a problem in Tree 2 as we've recently found out how to deal with them with regards the incubator, though this is still being discussed :)