From your OP I understand it, that there is no interaction in between that group and you, nor there is anymore than assumptions of why they decide to speak that language.
Given that, I'd rather provide you a frame challenge on this one.
I can't provide any citation on that, but I can give you a handful of examples what to keep in mind, when considering your approach.
As I can very well identify my self as one of these foreigners.
And I have multiple scenarios in my mind here, where your without doubt, well meant suggestion would most likely transport the wrong message.
First of all when I had been in a foreign country, I really loved to learn their local language. I learn by failing. And hence I try to make use of any opportunity to practice that language. When being with others sharing my mother language it is already hard enough encouraging others just to respect that I'd like to practice the language. The group could have come up with an agreement to practice the language at certain times or places, where it might fit. If you engaged them and told them they don't have to, you would encourage those who don't like it anyways (or discourage some not feeling as comfortable with being bad in something and making them insecure) to switch back to their mother tongue. And me as the one having initiated the idea would have been very disappointed.
Also it might be that they decided for reasons of politeness given by some cultural background. But also in that case, your request of them stop being polite could possibly come over as intimidating at best, as their pronunciation apparently reveals assumptions of their particular language skill, or maybe even as rude. Better just appreciate their attempts to be polite in that case.
And last but not least thing coming into my mind, they might possibly also just do it, to be able to make new contacts. Passively inviting you or others to join their conversation if you feel like having something to add. This is something I do sometimes, too in such a setting. I appreciate others adding their 2 cents onto discussions I have. And for that reason I usually use a language, that is most likely, most people being present could communicate in. In this case, as well. Your suggestion, despite being meant well, most likely would come over as very discouraging and rejecting.
I am not saying there is no way to do what you ask for with your stated goals. But I couldn't come up with a way my self and rather wanted to provide you an idea of what to consider, how you could be perceived, IF you go to tell them, what you ask for here.