Complex sentence: Since 2000, the teaching of Welsh has been compulsory in all schools in Wales up to age 16, and that has had a major effect in stabilising and to some extent reversing the decline in the language.
Split into simpler sentences: Since 2000, the teaching of Welsh has been compulsory in all schools in Wales up to age 16. That has had an effect in stabilising and reversing the decline in the language.