That's the official adjectival form according to the Myanmar government. However, in practice I've never seen it used (even in Myanmar). People tend to either s

That's the official adjectival form according to the Myanmar government. However, in practice I've never seen it used (even in Myanmar). People tend to either say Burmese or simply use Myanmar as both noun and adjective, as in, "the Burmese language" or "the Myanmar language".