Internationalisation: multiple locale/language support

[Heliodex]

Describe the problem

Currently, Mercury Core only supports English as a language and as a locale for date/time formatting.

Looking back in the repo history there's an ancient commit c4b207d where I was trying out i18n with Inlang, though this wasn't completed due to other concerns at the time. There seem to be loads of i18n libraries for Svelte/Kit with varying levels of implementation difficulty, and SvelteKit is considering implementing their own solution as well.

Describe the proposed solution

@lIlIIlIlIII got further than my attempt in lIlIIlIlIII/MercuryCore/i18n with the svelte-i18n package, but stated that this was unlikely to be merged. We should consider whether we want to go with svelte-i18n, Inlang/Paraglide.js, any other library that has good support for self hosting and works on the server side, or a custom solution based on SvelteKit's builtins.