Should we teach kids in school how to speak languages which they're more likely to encounter?

Likely because they are useful languages internationally and, more importantly for schools, there is a greater number of qualified teachers for those languages. Students who want to take more ‘niche’ language GCSEs often are already fluent to some degree and don’t get taught it in school. They have to have someone come in to do the exams for them. Schools are lucky to get French, German, and Spanish teachers. Having a Russian, Polish, Chinese etc is an ask and a half for our already struggling school system. Not to mention it’s likely only a couple of students would take them.