Could you speak english when you were six или can you speak english when you were six?

Could you speak English when you were six?

could - II форма от can, значит, нам нужен именно этот глагол)

Could you speak English when you were six?
Can — это "можешь" (можете) или "умеешь" (умеете), то есть настоящее время, а could — "мог" (могла) или "умел" (умела), то есть время.

Поэтому первый вариант.