I speak English and Punjabi. I use English in lessons and most of the time when I'm talking with my friends, (even if they're also Punjabi-speaking) though I might use Punjabi to talk about something like RE. When I talk to friends about football I use English, but if I'm discussing a family wedding I'll use Punjabi because my ideas and thoughts about weddings and stuff are in that language, it's like a part of me that's Punjabi.
By the way, the right way to say the first bit of the word 'Punjabi' is halfway between the 'u' sound and the 'a' sound, so sometimes you'll see it written as Panjabi.
When I go home I switch between the two with my Mum and family. My Mum had a good education in India and of course she's a teacher, so she speaks and writes English really well. She always uses Punjabi for family matters like telling me and my brothers off, discussing relatives and visits, and stuff like food and clothing. I also use Punjabi when I talk with one of my uncles, who is less good at English than my Mum, and whenever I go to the gurudwara.
The Punjabi of the Sikh holy book is really old fashioned, so I don't really understand it, but I can understand the prayers - just as well really since I can't see how I could be a Sikh if I lost that. For me, my other language is how I keep in touch with my roots. This is what Punjabi looks like when you write it
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