1995: Finland, Legal Recognition of Finnish Sign Language
Finnish Sign Language was recognised in the constitution in August 1995:
Section 17 - Right to one's language and culture [...] The rights of persons using sign language and of persons in need of interpretation or translation aid owing to disability shall be guaranteed by an Act.
https://www.eud.eu/news/what-finnish-sign-language-act/
Source: https://www.eud.eu/news/what-finnish-sign-language-act/