Ok, moved from another thread as it seems to have got people going!
DS is two and speaks quite a lot, but only in Welsh.
I live in a Welsh-speaking community, I'm a native speaker and Welsh is my first language (in fact I'm a lecturer in Welsh lang&lit), my entire family are Welsh. DS attends a Welsh medium nursery 2 days a week, and is cared for by my mother 2 days a week. And me the other days! None of the carers speak English with him.
My DH has learnt Welsh to near-fluency, and only speaks Welsh with DS: it gives DH a chance to improve (slowly, with an nonjudgmental speaker ) and has given him a massive confidence boost when it comes to it.
I am not teaching DS any English at all, and I never speak English with him. DS will learn English quite naturally, mainly from the television, or from hearing it around when there are people who don't speak Welsh. It's how it was with me and my English is of a very high standard (no doubt there will be grammatical errors in this post now - but I have an Oxbridge PhD so it can't be all bad).
PILs are not Welsh, live 250 miles away, and have expressed sadness that 'they can't communicate with him'.
They learnt to say hello and thank you in Nepalese when they went on holiday, but despite knowing me for 10 years and my family for 6, they have never learnt any words of Welsh at all, not please or thank you, and say it's pointless because it's a dead language, and it's not an useful language.