[wip] Lazy load language files

[louislam]

Description

Load the language files on demand, rather than loaded all language files at startup.

No significant improvement. The page load speed should be faster theoretically, as the compiled index js file is smaller now.

Type of change

• Core

Tasks

• Load the language file at the first page load.
• Settings' menu words are not reactive?

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 test it
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)