Avoid requests for non-existent locals while loading localized diagnostics

[mjbvz]

Bug Report

🔎 Search Terms

• Web / webworker
• server

Repro

1. Using a build of TS Server running on web: https://vscode-web-test-playground.azurewebsites.net/?enter=true
2. Open a TS file
3. Open the developer tools and view the network log

Bug

Notice that that are two requests being made that result in 404s:

Request URL: https://vscode-web-test-playground.azurewebsites.net/static-insider-ed6c343edb720d91ee0859e566c84b5687f5a9d5/extensions/typescript-language-features/dist/browser/typescript-web/en-us/diagnosticMessages.generated.json

And

Request URL: https://vscode-web-test-playground.azurewebsites.net/static-insider-ed6c343edb720d91ee0859e566c84b5687f5a9d5/extensions/typescript-language-features/dist/browser/typescript-web/en/diagnosticMessages.generated.json

As far as I can tell these files do not exist, so the 404s are expected. However it would be nice to avoid the requests entirely in this case since we should know there are no bundled localizations for en or en-us

[mjbvz]

VS Code is launching TS server here using --locale en-US, which is what triggers this. A few thoughts on fixing this:

1. TypeScript internally keeps a list of valid locals and avoids making the request / file exists check if the requested locale is not on the list
2. VS Code stops sending send along en / en-US to typescript since it is the default (this would not fix cases where a user configures their local to an unsupported one)