bad ?lang= in GET request should return an error (original) (raw)

The API is pretty lax about lang values, and will try to return a matching localization for an addon (etc) if available. In api/v5+ we return the requested lang as null if it's not found - if the lang value is garbage data (defined containing other than alphanumeric characters + _, -) we should return a 400 instead of returning a normal response (in api/v5+ because breaking change).

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