Added Finnish language translation

[JonneSaloranta]

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Description

Added a language translations (Finnish)

Type of change

• Other

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

[JonneSaloranta]

Now with 100% Finnish translations.

[louislam]

Will be merged after #2767, thanks

[louislam]

Just realized that it had been merged previously, but it should be fi instead of fi-FI?

https://github.com/louislam/uptime-kuma/blob/master/src/lang/fi.json

[JonneSaloranta]

Interesting... did not think to check the lang file. How come some other languages are using xx-XX format and some are not?