Language groups: one combined pair plus separate languages (EN+PT and RU)

Reported by Vladimir via e-mail (EFKey 2.1.1, build 400). He types in English, Portuguese and Russian:

"Ideally I would like to have a combined mode for EN-PT (as they both are latin) and a separate RU with a quick switch between these two modes. Also, now the combined mode for EN-RU does not make any sense because user can't see both latin and cyrillic keys anyway."

Where it stands today

• Multilingual typing is capped at 2 languages (memory). With three languages you have to keep Multilingual off, and the switcher cycles EN → PT → RU, each alone. A combined EN+PT is only possible by removing RU from the list.
• Nothing stops you from combining two languages with different scripts (EN+RU). The layout only ever shows one script, so the second language's corrections are useless there, and the switcher gets an extra "both" step that does nothing useful.

Proposal

1. Language groups. One combined group of up to 2 languages with the same script (EN+PT) plus any number of separate languages (RU). The switcher (spacebar swipe, language key, picker) cycles EN+PT → RU → EN+PT. Only the active group's engines are loaded, so the memory cap stays.
2. Same script only for the combined group. When enabling Multilingual with two different scripts, warn or don't allow it. Cheaper first step even without groups.

Touches

• language cap (more than 2 in the list, max 2 loaded at once),
• solo mode and the switch cycle,
• spacebar label and the language picker,
• the language reported to iOS (multilingual for a combined group, the single language when a separate one is active),
• Settings › Languages UI (group editor).

Medium-sized change.