Localize built-in UI and turn-by-turn instructions

The built-in UI is English only:

• the search control ("Search Maps", "Find Nearby", category names, kind labels like "Café");
• the Offline Maps panel ("Download New Map", "Only Use Offline Maps", status text);
• turn-by-turn instructions (services/instructions.ts: formatInstruction, spokenInstruction) and the voice.

Apple localizes all of it.

Scope

• A small message catalogue with an options.locale (defaulting to navigator.language) and English as the fallback.
• Instructions: port the phrase tables to a per-language structure. OSRM's osrm-text-instructions is a good reference for grammar per language.
• Distances already follow the locale's units. Also format numbers and dates (the offline list's "Downloaded Sep 23") with Intl.
• Voice: choose a speechSynthesis voice for the locale.
• Start with a second language to prove the structure (e.g. German or Spanish).