As docs say if stream goes online in response I can get language of stream. Is this field all time filled? Or maybe can be NULL?
Depends if the broadcaster has set their language or not. it’s not very often set by the streamers generally speaking.
iirc there are some edge cases where the webhook may fire before it the language field is populated in Twitch so the hook fires without it present in the data.
Do I understand correctly that if the broadcaster did not specify the broadcast language, then the corresponding field will not be in the server response?
It should be null as apposed to missing
Furthermore in addition to what Barry said, If the broadcaster later sets or changes their language, another webhook event will be fired with the language field populated. This could be combined with other changes that happen at the same time, like game changes or title changes.