I want a REST API V1 or V2 call to get count of boards having particular language

Lithium has separate category for each language.  E.g if language is Japanese there would be a top level category for that.You can check your community structure for that.

SELECT count(*) FROM messages WHERE category.id = <category.id>

Documentation:

https://community.lithium.com/t5/Developer-Documentation/bd-p/dev-doc-portal?section=commv2&collection=messages#constraints-category.id