Ivor, the question you ask should be, "What advantage does learning maths bring to anyone?"OK, let's get back to facts then.
What advantage does learning maths in Welsh bring to anyone?
People who seek Welsh medium schooling for their child usually start at 3 in state provided nursery classes (2 in play groups before that), so the language is well past the 'confusion between the two' stage. The school is totally immersive. By the time a teacher is bisecting angles or doing algebra, the language he/she uses to do it is a nothingburger.
As an aside... I was the head of an English medium school in Wales, which had 24 nationalities and 28 languages or dialects. Those kids were supported in their language, but their education was totally immersed in English. I can prove that if we had them before 7 years old, they suffered no deficit at all.
So it is with Welsh medium schools.