So i’m planning to create font that supports my language which is extended cyrillics. What kind of actions i should take before making it?
Are you asking if it’s OK to modify an existing font for your own use or are you planning to redistribute it?
This largely depends on the end user license (EULA) that came with the font (they all have them). Some font owners allow others to modify their fonts for personal use. Others don’t. Fewer still allow people to take their work, make changes to it and redistribute it. There are some libre or open source fonts that are publicly open to anyone changing them and doing anything they want with them. Again, though, look for the EULA to see what’s permissible.