[BUG] [Singleton Service] Don't support version fallback in getSupportedLanguageList API

[nannany00]

Describe the bug
Version fallback feature is added for related translation APIs, this feature mean: when the product upgrade to a newer release, and the translation is not ready for the newer release, Singleton Service will return the translation of previous version as fallback.
But now, the language list if product get from Singleton Service still is null.

To Reproduce
Steps to reproduce the behavior:

1. Start the Singleton Service build
2. Go to API: GET /i18n/api/v2/locale/supportedLanguageList Get the supported language list
3. The Testing data:
   Testing/1.0.0
   When try to get translation for Testing/2.0.0, it can return well, but for language list, it will return 404 error as below:

{
  "response": {
    "code": 404,
    "message": "Failed to get locale list for Testing/2.0.0",
    "serverTime": ""
  },
  "signature": "",
  "data": ""
}

Expected behavior
If the translation can be returned well, then the language list should also be returned well.

Screenshots
NA

Desktop (please complete the following information):
NA

Additional context
NA

[tigershi]

With Colin's suggest, we need to schedule a meeting to discuss the version fallback content

[tigershi]

the version fallback APIs design
https://confluence.eng.vmware.com/display/GQ/API+-+Version+Fallback