My ex is Latin American and when I first moved to be with him him and his mum would sit and speak in Spanish when I couldn't understand a word - at dinner every night (after I had cooked for us all), when we went out for the day, when we just sat and chatted over coffee. It drove me INSANE but he was adamant he just couldn't speak English with his Mum as it was unnatural and the conversation couldn't flow.
It made me feel like shit, till I saw them around my father-in-law (American, no Spanish) and realised they had been doing this to him for over 15 years - just having private conversations with him sat at the table too, completely unintentionally - they spoke Spanish together for 15 years, why would they speak a language they're not comfortable in for someone else?
I've brought it up with them both, I have walked out of restaurants where I have sat in silence for a good hour with the occasional - oh mums just telling me this. But they cannot speak together in English - even if we start in English one of them falls back into Spanish within a few sentences and I don't think they even notice.
He tried to explain that his mum isn't his mum in English, she can't fully be herself or express her thoughts without having to really think about it, and often says not quite what she means (she always comes across as rude or brash) and that was that.
In the end we lived in a Spanish speaking country so I picked up the language. And - even though we both speak English and Spanish, we always speak in English alone together, because it is the language we know each other in, but can have conversations with his family/our friends in Spanish - but weirdly, even if we're in an all Spanish conversation, if we have to address each other (I'm just running to the toilet, what was his name again? Do you want another drink) will always happen in English!
The other things probably are a red flag, but the language thing could be about comfort. I have friends who speak both but I can only speak to them in one, because to an extent my personality changes slightly with each language - and whatever language I met them in, is the language I am more comfortable speaking with them in future.
Speaking from experience, being forced to speak to someone you're comfortable with in a language you're not, is awkward - even if it is for someone else benefit. IMO its much better when she is with her friends she speaks spanish (and if you are more comfortable in Afrikaans with your friends, go ahead) and translates for you bc it basically puts out the whole group, instead of one person.
If she can't translate or you get bored - stop going, it wont affect your relationship you'll just have your time and hers - and I expect at a big event like a BBQ you wouldnt be side by side all night and you could speak to anyone in whatever language you want to.