Fix calculation of "limited translations" to not include en_US fallback
Summary:
Broken by me in D25695 last year. Sigh, I really wasn't on my A game then.
And also mark locales with zero translations (like "English (Canada)" which doesn't have an upstream translations file) as limited too even if they are varieties of English.
Depends on D26559 to work.
Test Plan:
With the downstream translation extension installed, see more languages (like "Asturian") in "Limited Translations" on http://phorge.localhost/settings/panel/language/. Note that this section is only shown in developer mode (http://phorge.localhost/config/edit/phabricator.developer-mode/ resp `./bin/config set phabricator.developer-mode true`).
With the downstream translation extension installed, see "Tarantino" not in Limited Translations (it has < 512 strings by itself but >512 strings if you count the Italian fallback)
Without the downstream translation extension installed, see all of the locales upstream in "Limited Translations".
With or without the downstream translation extension installed (translatewiki.net doesn't support en-CA), see "English (Canada)" in "Limited Translations".
Reviewers: O1 Blessed Committers, aklapper
Reviewed By: O1 Blessed Committers, aklapper
Subscribers: aklapper, avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26560