Localization

[oleganza]

I suggest we add locale/region tags to make messages from the app properly localizable.

• On the initial reply, the wallet should send a language+region tag per RFC 5646 (e.g. fr-CA for French in Canada).
• Add locale+region to the "account_changed" RPC message and send it to all the connect apps when the wallet changes its locale.
• Specify that the apps MUST use the locale and region when formatting any human-readable text over the TON Connect bridge.

[siandreev]

Now the protocol does not provide for displaying custom messages to the user. I think we can come back to this when the need arises.