Improve i18n utility, add English fallbacks

[JSKitty]

Abstract

This PR adds a well-needed cleanup to the way devs interface with the i18n system.

• We now have a tr() utility function, for in-line translations of strings with variables.
• The i18n system now uses the English file as a fallback if a key is missing.
• Translation switching is now a per-key process, rather than entire file.

This means a few small changes to how we previously used it, for ex:

Translating variables in-line is now super easy, i.e:

// i18n key `hello` is "Hello {user}!"
// Which becomes "Hello Bob!"
const user = 'Bob';
const str = tr(translation.hello, [{ user: user }]);

Alerts no longer have the additional variables param, devs are now expected to use tr() in-line to translate alerts on-the-fly.

Before

// With variable
createAlert('success', ALERTS.SUCCESSFUL_TX, [{ txid: txid }], 5000);
// Without variable
createAlert('success', ALERTS.SUCCESSFUL_TX, [], 5000);

After

// With variable
createAlert('success', tr(ALERTS.SUCCESSFUL_TX, [{ txid: txid }]), 5000);
// Without variable
createAlert('success', tr(ALERTS.SUCCESSFUL_TX), 5000);

Previously, dynamic JS-selected translations would return '' or undefined to the user in the GUI, if the i18n keys were missing or unset for their language; with the English fallback, this is no longer the case, and users will never be presented with undefined or empty text now.

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Testing

To test this PR, it's suggested to attempt these user flows, or variations of these:

• Change your language (multiple times across a few testing sessions and wallets).
• Import or Create a wallet.
• Test various parts of the wallet, ensuring you see no suspicious Notifications or Text which is either empty, or says undefined. (Send TXs, try causing errors, change settings, create a proposal on Testnet, etc).

If any errors are found, the PR works unexpectedly, or you have viable suggestions to improve the UX or functionality of the PR, let me know!

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[Liquid369]

tACK 16aebc8
Continually getting better for the i18n system translations 🔥

[Liquid369]

tACK cc87b58
Attacked this a few times, it looks much better now and well continue to overhaul the i18n!