Recognise en-US and other English variants as the source in check

[arvida]

Fixes localheroai/localhero-ai#827.

Without a localhero.json, check treated only a plain en as English. A project whose English file is en-US.json (the most common JavaScript naming) fell through to "the language with the most keys", so every locale was compared with, say, Arabic. In a 40-repo run over popular open-source apps this hit 9 repos, e.g. formbricks → ar-EG, umami → az-AZ, mealie → af-ZA, lobe-chat → zh-CN.

Now: a plain en first as before, else en-US/en_US (any case), else the first other en-*/en_* variant. The gettext and .pot rules keep their precedence. The info line reads "English is present" instead of "en is present".

Verified

• Tests first; full suite passes, lint and build clean.
• Built CLI on formbricks, umami, mealie and lobe-chat: source is now en-US and files pair correctly (real key counts, no "Not checked"); open-webui, cal.com, saleor-dashboard and lago-front pick en.
• Reviewed (code-reviewer agent; a Codex run timed out): no blockers. It confirmed file pairing escapes the locale and matches it as a whole token, so an en-US source pairs en-US.json with de.json and locales/en-US/ with locales/de/.