fix/i18n-accept-language-parsing

[Sushma-1706]

Motivation

Improve robustness, correctness, and standards compliance of Accept-Language header parsing used for language preference selection. The previous implementation did not strictly validate tokens, lacked proper normalization, and did not enforce q-value validation consistently.

Description

This update strengthens the parseAcceptLanguage and getAcceptedLanguageCodes helpers to ensure predictable and standards-aligned behavior.

Improvements

• Trim and ignore empty language entries before parsing.

• Use a fully anchored regular expression (^...$) to prevent partial or malformed matches.

• Support primary language tags (2–3 letters) with optional subtags (e.g., fr-CA, en-US).

• Support optional whitespace around ;q= (e.g., EN-us ; q=0.9).

• Default weight to 1 when q is not specified.

• Validate q values to ensure they are numeric and within the range [0, 1]; skip invalid entries.

• Normalize language codes and subtags to lowercase.

• Sort parsed languages in descending order by weight.

• Ensure type safety using a TypeScript type guard when filtering invalid matches.

Testing

• Added comprehensive tests covering:

• Correct parsing and sorting by q weight.

• Whitespace tolerance and case normalization.

• Proper handling of subtags.

• Rejection of malformed language tokens (e.g., @@, *).

• Rejection of invalid q values (e.g., q=abc, q=1.5).

• Correct output ordering and normalization in getAcceptedLanguageCodes.

Result

These changes improve standards compliance, security, and deterministic language preference resolution while maintaining backward-compatible behavior for valid headers.

[leftmostcat]

I'm a little concerned about this as I'm not sure parsing Accept-Language is something we should be doing by hand, and this adds a fair bit of complexity to the process.

I'll leave it to monkey whether to go with this approach, but if it were to be accepted, I would at least want to see references in comments to the relevant RFC to explain what's happening and why.