I am from North Wales, I speak Welsh (self-taught) and I have lived in Cardiff and I now live in Merseyside. I also have a degree in Linguistics and I am a secondary English teacher in a Welsh school. The perception of Welsh in South East Wales is vastly different to the approach we have in the North - nonsensical discussions on the 'purpose' of the language don't really happen up here. Our languages merely co-exist and flow together, whether you're from a bilingual family or English-only.
However I'd like to clear a few things up on the Welsh education system:
- All Welsh kids (in English medium AND Welsh medium schools) study English first language and most achieve GCSE English Language and GCSE English Literature.
- Those in English-medium schools receive Welsh second language and all must study at least the half-GCSE Welsh Second Language (short course). Many schools opt to do the GCSE Welsh Second Language (full course) as an option. These second-language courses are taught like any other modern foreign language.
- Those in Welsh-medium schools receive Welsh first language taught as an equivalent to English. Most kids are then entered for GCSE Welsh Language and GCSE Welsh Literature.
By law, all pupils in Wales must study Welsh until age 16 through one of the channels mentioned above. Disapplying a pupil from GCSE Welsh is the same procedure as disapplication from GCSE English and is rarely done.
Subsequently, those pupils who go to Welsh-medium schools have more GCSEs upon leaving secondary education. There have been many, many studies on the effects of bilinguialism, and the majority indicate (this is not specific to Welsh) bilingualism benefits language skills in both languages. Moreover, most of the world's population is multilingual, and much of the Anglophone world is in minority being monolingual. You will find that the majority of classes in Welsh-medium schools take a bilingual approach, especially with subject-specific jargon as we understand that the vast majority of higher education takes place in English ("sandwiching" is the educational term we use for bilingualism term introductions).
I always teach my pupils that English has an absolutely shocking spelling system (pidgin Anglo-Saxon-Latin-French-British) but incredibly easy grammar; Welsh on the other hand has exceptionally easy spelling (like Spanish), but more difficult grammar. Incidentally, I have come across pupils who are dyslexic in one language but not in the other. Languages, their social histories, their evolution and their psychological bases absolutely fascinates me. If we were purely going on language 'usefulness', why are we not all learning Mandarin Chinese, Arabic or Hindi, which indeed have far greater number of speakers than German or French? English is a lingua-franca; it is an essential skill in a globalised world, but we must not be complacent and accept that English is not the only (nor the largest) lingua-franca. People who have absolutely no understanding of the Welsh (or any other minority) language, its history and culture provide little credibility in language-bashing rants I'm afraid.
As for switching languages mid-conversation... this is called code-switching, which occurs in multilingual speakers of any language. It is an extremely complex (so much so it is hard to document and study) sociological-psychological phenomenon that is determined by a mixture of external factors. You will find most bilingual speakers speak to each person only in one language (the one in which they first met and talked). To change that is extremely 'odd', almost seeming taboo, and an almost unbreakable maxim. Codeswitching is so ingrained, bilingual speakers are often unaware they are doing it and it can occur in the strangest of moments. Where there are more people of one common language, people will of course accommodate to a shared code, but it is often difficult to sustain where two speakers share close and ingrained bonds in one language.