fix(spas): use cased locale for homepage, not lowercase

[caugner]

Summary

Problem

On https://developer.mozilla.org/en-US/, the menu links point to en-us, e.g. https://developer.mozilla.org/en-us/docs/Web instead of https://developer.mozilla.org/en-US/docs/Web. This causes an unnecessary redirect.

Solution

Build the homepage using the properly cased locale (en-US).

(While at it, this also makes use of DEFAULT_LOCALE where possible.)

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How did you test this change?

1. On main, I ran yarn build:prepare.
2. Ran git add -f client/build.
3. Switched to this branch.
4. Ran yarn build:prepare again.
5. Looked at git diff --word-diff client/build and saw the following:

[caugner]

@LeoMcA In 648de18, I typed the params and fixed the m2h call.

[LeoMcA]

lgtm!