Always translate server-side

[dktapps]

Problem description

We currently rely on the client to translate strings presumed to be present in the vanilla files (anything not prefixed with pocketmine.).
We also don't take any information from the client about its selected language.

This is problematic for a few reasons

• Complexity
• Some keys which aren't prefixed with pocketmine. aren't present in the client (a list of those as of 1.21.60: https://github.com/pmmp/Language/blob/stable/.github/workflows/known-bad-keys.json)
• Non-vanilla translations are not translated to the client's selected language

Proposed solution

We should use the locale provided in LoginPacket to decide the server-side language used to translate messages.
However, this is tricky due to the different codes used (we use 3-letter codes like eng while the client uses locale codes like en_US).

This is really two issues in one, but we can't do fully server-sided translation without tying it to client-provided locale.

Alternative solutions or workarounds

No response