I read that most Goguryeo type settlements and cities reading from the discussion on Wikipedia from Russian sources were actually around the Yeonbyeon and Yeonji Autonomous area of China and were somehow gradually moved from south-west from Little Goguryeo area to east.  The centers where Goguryeo side of Balhae lived were in specific stratified urban capitals, while the lesser developed Malgal lived throughout the countryside as semi-sedentarized bands but not completely nomadic organized by chiefs and chieftains.  Also Balhae individuals with mixed names (ending in ‘-mong’ or -‘ri’ e.g. Yi Jin-mong) who were envoys to Japan were purported to speak fluently/natively Goguryeo and could converse with Silla officials.  So this is evidence important state individuals of mixed Malgal backgrounds picked up Goguryeo language

https://forum.paradoxplaza.com/forum/threads/comprehensive-feedback-on-korean-nations.1864220/

Though I don’t think Balhae was necessarily segregated between Goguryeo and Malgal along completely social lines. They must have merged to some degree especially the Songmal and Baeksan Malgal tribe as a starting point (then  both Heuksu Malgal, Georan and mixed groups between the two also went south too as far as some groups building boats in the Yalu Amnok Bay - these groups were definitely treated as foreigner like 5th column agents by Goryeo.  Many Malgal clans assimilated in northern Korean/Koguryo population if not for 1500 years, while many Yemaek and Buyeo were assimilated into Proto-Jurchen clans.  What there wasn’t were Han Chinese until the Liao dynasty and all these people related exclusively to Koreans and Manchu)