Slovak language not working due to incorrect filename

**Version:**
 - listmonk: v2.5.1
 - OS: Ubuntu

**Description of the bug and steps to reproduce:**
It seems that the file is named incorrectly, according to <select> box in administration of Listmonk, the Slovak language is "sk-sk", however the file is "sk.json". So either rename the file to "sk-sk.json" or change the option in the select box please.

Thank you
