Set language via localStorage if set

[eyeseast]

Closes #784

This sets the $locale store in the top layout using localStorage, if we've set it.

I'd like to figure out a way to set it even earlier, because in this version, we're loading the English JSON file and then the other language, but this is a good-enough fix for now.

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Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	24.22%	8151 / 33646
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🔵	Functions	43.23%	195 / 451
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File	Stmts	Branches	Functions	Lines	Uncovered Lines
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src/lib/components/navigation/LanguageMenu.svelte	0%	0%	0%	0%	1-74
src/lib/components/viewer/ViewerContext.svelte	0%	0%	0%	0%	1-217
src/routes/+layout.ts	0%	0%	0%	0%	1

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[eyeseast]

I'm going to merge this and then might try setting a cookie. Language preference might also be something we should store in the API, on the user object.