Further distinguish language code for Chinese (zh-tw and zh-cn)

**Is your feature request related to a problem? Please describe.**
Currently, accoding the language setting on Crowdin, Traditional Chinese is translated under [Taiwanese Mandarin](https://en.wikipedia.org/wiki/Taiwanese_Mandarin), which is using zh-tw instead of zh-hant at start. It should be sufficient at current state of development to have it to meant to be zh-hant's translation, though in technical reasons, this should be avoided, due to the fact that zh-hk and zh-mo has their own glossaries and terms that zh-tw does not have.

In addition to the context, current Simplified Chinese on crowdin is using zh-cn, which might also using lots of non-Singaporian or non-Malaysian glossaries and terms.

**Describe the solution you'd like**
It should be more technically reasonable to have zh-tw encodes to `繁體中文（臺灣）`, with flag of Taiwan. Correspondingly, zh-cn should be encoded to `简体中文（中国大陆 ）`.

**Describe alternatives you've considered**
None.

**Additional context**
Such differences is properly handled before, e.g. Deutsch (de) and Schwiizerdutsch (de_ch), so it might should have handled here as well for further clarification.

**Assignment**

- [ ] I'd like to be assigned to this issue and work on it myself

Start by tracing the localization language-code mapping used for Crowdin's zh-tw and zh-cn entries, comparing it with the existing de/de_ch distinction mentioned in the issue. Done means zh-tw displays 繁體中文（臺灣） with Taiwan's flag and zh-cn displays 简体中文（中国大陆）.

Written by the indexing model from the issue text.