bug: no fallback docs link on unsupported language

[pemassi]

What problem did you meet?

I am trying to add a new language(Korean) to Logto application (https://github.com/logto-io/logto/pull/1677). If I click the Docs from the sidebar in the admin console, it will open new page which is https://docs.logto.io/ko-kr. Then I will see the below image.

Describe what you'd like Logto to have

If there is no translation for that language, it should be a fallback to English for the developer.

[charIeszhao]

Yeah, that makes sense. Thanks for reporting!