> How could we count these L1+2 Russian and English speakers?

good starting point would be checking when English/Russian was taught in schools and how many people are alive from that years

it will overestimate prevalence of Russian (many people in Russia-occupied countries were quite deliberately and proudly ignoring this lessons or had no reason to use Russian since occupier was kicked out and forgot it), but would be useful to the first approximation

(not that I consider worth doing it)