Learning a second (or third or fourth) language is a great thing and learning grammar, reading and writing in a second language will be fascinating to a good percentage of gifted kids.

On the other hand maths and science that are below what you need are as boring in a second language as they were in your first language. Struggling to understand the teacher's explanation (or understanding it well in a second language) is not a substitute for an accelerated and enriched STEM programme.