Reads translated plural forms straight from a backend's .po files.

Why this exists

Gettext's runtime API cannot give us an untranslated-but-localized plural form. Gettext.Backend.lngettext/7 unconditionally overwrites the :count binding with the numeric n it was called with:

var!(bindings) = Map.put(bindings, :count, n)

So there is no way to ask it for the Spanish plural of "%{count} items" with the %{count} placeholder still intact - and the placeholder is exactly what has to reach the browser, because interpolation happens client-side with the real count.

Before this module existed, all_translations/1 worked around that by returning the raw English msgid/msgid_plural, which meant plurals were never translated at all.

Reading the .po file gives us the exact msgstr for every plural form, placeholders untouched.

Plural form selection

The browser has to pick a form from a list. Gettext identifies forms by a numeric index whose meaning is language-specific, while Intl.PluralRules in the browser speaks CLDR category names ("one", "few", "many", ...).

Rather than shipping a rule evaluator, we send a small probe table: a handful of representative counts and the gettext form index each one selects. The client asks Intl.PluralRules for the category of its count, finds a probe with the same category, and uses that probe's index. Both systems partition counts along the same linguistic lines, so a matching category implies a matching form.

Failure behaviour

Every function here degrades to :error or a safe default rather than raising. A missing, unreadable or malformed .po file must never take down a page render - it just means the untranslated strings are used.