... as it doesn't come at the cost of the national language.

What is a 'national' language? Who defines a 'nation'?

Can the Chinese Communist Party define that all Uyghurs, Tibetans, Mongolians whose territories are occupied are now Chinese that must be taught only in Mandarin?

Can Russia rule that Ukrainians in the occupied territories can't be taught in Ukrainian as this would "come at the cost of the national language"?

As one scholar from Turkey writes:

Numerous studies in linguistics, psychology, anthropology, and educational sciences demonstrate that monolingual education deepens linguistic, cognitive, and psychological inequalities among children. It not only places children at an academic disadvantage but also damages their relationship with their cultural identities. This leads to a loss of self-confidence at the individual level and to exclusion and alienation at the societal level. In this way, the education system becomes a mechanism that reproduces inequalities rather than eliminating them ...

Epistemic hierarchies, frequently found in colonial modes of thought, position certain languages as “central” and “universal,” while relegating others to “local” and “secondary” status ... Research, however, shows that children learn more rapidly in their mother tongue, that their conceptual development progresses more healthily, and that their cultural identities are strengthened. Mother-tongue education not only increases academic achievement but also enables children to feel equal and valued in the public sphere.

Lasting social peace is possible not where differences are suppressed, but where they are recognized and institutionally guaranteed. When the education system ceases to function as an instrument of homogenization and standardization and instead becomes a vehicle for pluralism, the principle of equal citizenship will acquire its genuine meaning ...

Emphasis mine.