The mother of my son-in-law is a delightful person, born in Europe. She is keen that our mutual grandson learn her native language, and the extended family, including my husband and I, agree. So whenever she minds the grandson, currently two years old, she speaks to him in her language. However, at family gatherings, she continues to speak to him in that language, in front of the rest of us. Essentially, it becomes their private communication. My husband suggests, tongue firmly in cheek, that we speak to the child in Pig Latin. To be serious, though, are we being too sensitive or can we ask her to converse in English when we are all together?
Ixnay. Ountcay ouryay essingsblay. (Pig Latin for "Nix. Count your blessings")
While we're on the topic of language, fasten your seat belts, put on your safety glasses (or 3-D glasses, whichever's handier), as your friendly neighbourhood advice columnist unzips his word bag, reaches in and whips out a word you might not hear that often: "idioglossia."
Definition: n. a private language no one else understands, most often between siblings (many twins have an "idioglossia").
Your son-in-law's mother has in effect turned whatever language she speaks into an idioglossia understood only by her and her grandson. Don't complain. Let me adumbrate (also from the word bag) some of the benefits about the situation you've described.
Let's just start with "she minds the grandson." You drop that in casually, as if it were the natural order of things, to be taken for granted. Far from it. Me, I'm lucky to have a mother and father in the area code (and another pair within striking distance). They have been absolutely critical in the care and feeding of my (now) teenage boys.
My father has always taken them on ceremonial edutainment-filled excursions. My mother answers her phone on the first ring, speeding over to our domicile in her convertible to save the day like some kind of septuagenarian superheroine.
I have never, ever taken it for granted. Be supremely grateful. She's helping your daughter and son-in-law navigate these lovely but (sometimes) difficult and exhausting years. Kids are a blessing but can also be a burden – one she's helping shoulder. (Not to mention simultaneously taking some of the babysitting pressure off you, as well.)
But to throw in free tutorials in a second language on top of it all? Everyone should be bringing this European grandmother foot baths and tiny glasses of sherry and/or milk of magnesia, and massaging her bunions. Kids that age are language sponges. Your grandson will learn everything about whatever language it is, including obscure words, without even trying, in a way that would cost untold time and effort, not to mention expense, later on in life.
Now, I suppose if she were an evil grandmother, she could be saying stuff to the kid like: "Your mother is unworthy of my son, that whole branch of the family are a bunch of bums, some day I'll take you away from all this hideousness, and we'll hide out in an undisclosed European location," etc. But, by your own testimony, she is a "delightful person," so that seems unlikely.
So, no, don't discourage this habit. Encourage it. Be happy and grateful your grandson has someone teaching him a second language, all while forming a special bond with him, 100 per cent free of charge.