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### What?
This PR adds `'sl'` to list of accepted languages, dedupes languages not
implemented from langs that have been supported, and adjusts the string
match for `'sl'` in importDateFNSLocale to the correct locale.
### Why?
To fix TS errors and runtime errors encountered while adding Slovenian
language to config, and then selecting it in `/account` view.
### How?
- Addition of `'sl'` to `acceptLanguages` array
- Change from `'sl'` to `'sl-SI'` in `importDateFNSLocale.ts`
Fixes#9504
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Co-authored-by: Jessica Chowdhury <jessica@trbl.design>
0c7e418dbc