Please can I sound a small note of caution?
I am upset by the behaviour of this worker, which would, as described be cause for a formal complaint, and possible disciplinary action in our system.
However, all the examples of others grans' multi-lingual children does not mean this one is OK. Multi-lingual children can have hearing & speech problems too.
OP has not said on what basis the worker is concerned, but gives the impression it is just some arbitrary idea. It does not imply confidence in the quality of the worker's assessment of the child, but it does need addressing properly, by someone who has the right knowledge & expertise. That is what the parents need to ask for.
I am very unsure as to the way Health Visitors in the UK operate now. In my day, in a very mixed area, I assessed multi-lingual children all the time. I also saw a number of families, unofficially, when they came 'home' for their annual stay with the British grandparents. They would pop in to clinic and ask me to do a developmental assessment based on UK guidelines for re-assurance. I'm not sure if there is that kind of leeway now, when I gather most areas have suspended clinics.