If you want to talk specifically about people in Belarus who fight for the right to use their native language, you could refer to them as to linguistic human rights activists. There's also the concept of cultural rights, which cover a broader set of national or ethnic identity features than just language.

I wouldn't describe such as "nationalists" at all if all they do is defending their own human rights without trying to do so at the expense of other national groups. I.e. a hypothetical party trying to promote Belarusian by banning other languages in schools or on TV would be "nationalist". A party trying to get the number of schools teaching Belarusian in agreement with the number of kids having Belarusian as their native language is not.

If you want to talk specifically about people in Belarus who fight for the right to use their native language, you could refer to them as to linguistic human rights activists. There's also the concept of cultural rights, which cover a broader set of national or ethnic identity features than just language. Ethnic nationalism could also fit, though it would not be perceived as "benign" anymore by many people.

I wouldn't describe such as "nationalists" at all if all they do is defending their own human rights without trying to do so at the expense of other national groups. I.e. a hypothetical party trying to promote Belarusian by banning other languages in schools or on TV would be "nationalist". A party trying to get the number of schools teaching Belarusian in agreement with the number of kids having Belarusian as their native language is not.