English is my second language, when I started learning I was always self conscious about my accent, so I would practice every day reading labels in the shower etc, and over time I developed really good fucking English, like nobody can tell its my second language now.
When I talk with people who have bad English, I will use broken English too, because it feels easier to me, even though I CAN pronounce things pretty much perfect, but if I try to talk after working out, people can't understand half of what I say. Accents can vary greatly, if I ever get angry pr tired, I get a really thick accent probably because I don't care about using that 5% extra energy to make my English sound perfect. It's really not strange at all to sound completely different on any given day. Sometimes I just don't care, and my English will sound twice as bad.
Also people underestimate how easy it is to pick up accents/slangs/etc just from hearing people around you talk, you probably do it a lot yourself and never realise. For most people who speak multiple languages that video is probably not strange at all