Delete the translation coverage tests and the tests of one language's
wording. Keep catalog validation (format, duplicates, placeholders, plural
forms, command ids) and the tests that use a language (registration,
plural rules, RTL order, glyphs, casing, font fallback).
cargo xtask i18n-report runs i18n::tests::translation_gaps_report, which
prints each language's missing count and first few keys and always
passes; xtask ci runs it after the tests.