I have a problem with JS HTML encoding
When I use non latin characters in window.alert() I get nonsense
If I use it in document html like ᄋ 弫 ᄋ ᄋ and so on then it is shown ok but when I put the same code encoding into the alert() box I get these ᄋ 弫 ᄋ ᄋ but not non latin text
How to fix it? What are the possibilities?

The default language that javascript supports is english. Other than english if you write any other language you will face problem.

The default language that javascript supports is english

Nope

The default display language of javascript is the default language of the users computer

Alert boxes are controlled by the client computers operating system so if the language and character set are not installed you will not get the correct characters.
With HTML the charset specified in the page will determine the character set which is probably why it works there then does not in the alert.