Tor Browser and friends are bad at picking locale

For instance, for the Tor Browser we have the pt-PT langpack. If we pick the pt-BR locale in Tails Greeter, we will currently fallback to en-US. We should instead set locale to pt and then Firefox will be smart enough to use the pt-PT langpack.

This is also true for Tor Launcher (and of course the I2P and Unsafe Browsers).

Feature Branch: bugfix/8122-better-browser-localization

Parent Task: #7953 (closed)

Original created by @anonym on 8122 (Redmine)

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