After MBS-14482 (pull request #3888), zh-hant and zh-hans can be in MB_LANGUAGES. Browsers often send a region tag for Chinese, such as {{zh-T
W}}, instead of a script tag, such as zh-Hant. For example:

Accept-Language: zh-TW, zh, en

These region tags do not directly match zh-hant or zh-hans in MB_LANGUAGES. If a tag in Accept-Language is in MB_LANGUAGES, the server
can select it. zh-tw is not, so the server tries the next tag. I18N::LangTags also adds zh as the parent of zh-tw. If zh is in {{MB_L
ANGUAGES}}, as on beta, a browser that prefers zh-TW gets Simplified Chinese. If it is not, the browser gets English.

The same occurs for the six region tags below:

The users can select their script in the language menu. But the first page that they see is in the wrong script, or in English.

Steps to reproduce

Prerequisites:

• The code of MBS-14482 (pull request #3888), without a fix for this issue.
• MB_LANGUAGES set to en zh zh-hant, and LANGUAGE_FALLBACK_TO_BROWSER set to 1 (the default).
• The zh_CN.UTF-8 and zh_TW.UTF-8 system locales.
• The installed translation catalogs (make -C po install) and the client bundles (./script/compile_resources.sh).

Send a request with the header and without a lang cookie:

curl -s -H 'Accept-Language: zh-TW, zh, en' http://localhost:5000/ | grep -o '<html lang="[^"]*"'

Expected: <html lang="zh-Hant", a page in Chinese (Traditional).
Actual: <html lang="zh", a page in Chinese (Simplified).

With MB_LANGUAGES set to en zh-hans, the header zh-CN, en gives English instead of Simplified Chinese.

Suggested fix

When the server reads Accept-Language, treat each region tag in the table as its script tag, if MB_LANGUAGES has the script tag. Put the script tag right after the region tag, so that it comes before the zh that follows.

Acceptance criteria

• If its script tag is in MB_LANGUAGES, each of the six region tags selects that script tag.
• An exact match comes first. For example, a server with zh-tw in MB_LANGUAGES still selects zh-tw.
• If the script tag is not in MB_LANGUAGES, the selection does not change.
• The quality values of the header keep their order. For example, a header that gives en the quality 1 and zh-TW the quality 0.8 selects English.
• A language in the lang cookie still comes before the header. The language menu sets the cookie to the script tag, so the cookie needs no change.
• Script tags in the header, such as zh-Hant-TW, and other languages keep their current behavior.