The choreography of conversation when not everyone understands the language

From David Eddie: 

Every spring my mother-in-law arrives from Europe. While she stays in her own home we see her often, usually for meals and then a four-day visit to the cottage with us. Although she speaks English very well, she seems to feel we should all be learning her language and accommodating her, to the point that she will often speak her language at these meals. So instead of saying “pass the butter” which is hardly a complicated matter in English, she will revert to her own language and then she hooks in my husband and they begin talking and no one has a clue what they are saying. I know it’s a power grab so she can control the conversation and cut me out but my husband is afraid to stand up to her because she has quite a temper, and because he says that at 78 you get to do what you want to. This causes untold friction in my family and, judging from the number of mixed marriages in Canada, for many other families, I am sure. Is it rude to speak a foreign language in front of people who don’t understand?

My credentials: I was born into a bicultural family, where some family members don't speak the local language very well, and still others choose to talk among themselves in the heritage language despite being functionally bilingual. I am fluent in the local language, but for most of my life I understood nary a word of the heritage language.  (I understood it as a toddler as well as a toddler understands anything, then lost it when I began school and started learning it in adulthood, but I'm still nowhere near fluent and can  follow along only sporadically.)  So I grew up immersed in this situation, but nearly always as a unilingual party who didn't understand half of what was being said.

In this capacity, I propose that the best approach is for the husband to translate the conversation for his wife.  He doesn't have to do every single word, he can just say "Mum's asking about our vacation, so I'm telling her the story about the elephant and the guy with the hat." If his mother's receptive English really is fluent, perhaps he can even respond to her in English so his wife can follow along, and his wife can participate in the conversation too. Then when his mother responds in the heritage language, he can translate her statements.

While all this is happening, the wife should feel free to participate in the conversation in English even if she doesn't understand every word that's being said.  For example, after the husband says "I'm telling her the story about the elephant and the guy with the hat," the wife could chime in with "And make sure you tell her what the weather was like that day!" - regardless of whether he's already told her that part. 

As an added bonus, if the mother can in fact express herself in English as easily as LW thinks she can, she will naturally begin using more English in this context.  It might be to speed things up, but it quite often even happens through normal code-switching patterns.

This will achieve the same result but make the mother feel like it was her idea, all without having to have an awkward conversation trying to convince her not to converse with her child in the language in which she naturally converses with her child.