"Es i i ysgol cyfrwng Cymraeg."

Translation: I went to a Welsh-medium school.

[DavidLangdon0]

Why is it not "gyfrwng"?

[DavidLangdon0]

In answer to Mizinamo, my point is that in the sentence Duolingo gives there is NO lenition, i.e. "ysgol" is followed by "cyfrwng". So I am asking why this word is NOT mutated to "gyfrwng". No doubt there is an explanation, but I'm asking what that explanation is.