Flutter internationalisation uses slang

Internationalisation now lists slang as the Flutter tooling, with .i18n.yaml catalogues.

• Slang supports ICU-style concepts such as pluralisation and interpolation. The mobile app’s catalogues use slang’s syntax rather than ICU MessageFormat syntax.
• A new Flutter section shows the .arb example in slang YAML, lists slang’s modifiers, and sets key-naming rules.
• The page summarises what the mobile setup provides: compile-time key safety, a single source catalogue, and CI checks for stale, unused, and missing translations.