As far as flutter support on Couchbase Lite, there are several community driven initiatives. While we do not officially support the third party plugins, there is a plugin developed on top of our CBL-C API from a community partner that will be GA with our next 3.0 major release of CBL- I would recommend checking the plugin out (CC @gabriel )
if numbers are important then Scala is a language with less popularity than flutter right now, and there is an SDK for Scala right ?
BTW, from your statement above, sounds like you are discussing server SDK and not Couchbase Lite SDK which is what this thread is about. Scala is supported on our server SDK and not couchbase lite. The list of supported languages on CBL is available here