What is the proper response to "Excuse my language"?

Solution 1:

A proper response could be...

...Moving forward, let's try to keep a civil tongue

keep a civil tongue

to speak decently and politely

[The Free Dictionary]

By including yourself in the conversation (let's try to...), you show that you are not being judgmental or nitpicky (even though you'd like to be) and are offering this as a general advice.

Moreover, if you are observing this behavior (of rude language) repeatedly, you can do away with the politeness and just ask them to watch their mouth!

Solution 2:

Because the question involves etiquette as much as linguistics, I consulted The Emily Post Institute, a leading American authority on manners and civility. Their advice is to say, politely, "Excuse me, would you mind toning down the language? Thanks!”

(The person posting on the Post site suggests that if one feels uncomfortable, one should say nothing.)

I find that I use this approach with my adolescent students, whose energetic invective and scatological commentary can sometimes overstep the bounds of proper decorum. They generally respond well to my "Tone it down, folks!" even when I utter the phrase in a judgmental, preachy and magisterial way.