[Koha-translate] Add new language

Hi there, This is a request to add below language to all koha projects Language: Sinhala Native name: සිංහල Code: si (do not add as si_LK) Pootle account: helabasa Projects 1. Koha 2. Koha manual 3. MARC21 SQL Files 4. Terminology Good day Sent with [ProtonMail](https://protonmail.com/) Secure Email.