Hi,
'It is not necessarily these small languages that are about to disappear.'
I can understand the SENTENCE STRUCTURE.
Consider this simpler and more concrete example.
Tom's car won't start.
It is not necessarily the battery.
It might be out of gas.
The underlined sentence means
'For the car not to start, it is not necessary that the battery is dead'
'The battery may not be the problem'.
In your example, here's the general meaning.
Some languages are going to disappear.
It is not necessary that the small languages disappear.
It may be the medium ones or the big ones that disapper.
What do the words 'not necessarily' refer to?
I see the words 'not necessarily' as an adverbial phrase' modifying 'is'.
Best wishes, Clive