Nice one. Thanks for this 🎉

I assume https://github.com/matomo-org/matomo/blob/4.13.3/core/Common.php#L945-L950 the extractLanguageCodeFromBrowserLanguage method needs to stay and only return 2 letter code? Do we maybe need to check the users of that method if they maybe also need to call a different method or maybe we did already? (I see you already changed it in few places). Is that that in tracking we only log 2 character language code?

Be also good to add various tests.

I've got a few potential cases below. May be able to provide next week more if needed

            ['en', 'en-US,en;q=0.8,de-DE;q=0.6,de;q=0.4'],
            ['de', 'de-DE;q=0.6,de;q=0.4'],
            ['zh-cn', 'zh-CN,zh;q=0.9'],
            ['fr', 'fr,fr-FR;q=0.8,en-US;q=0.5,en;q=0.3'],
            ['fr', 'fr-FR,fr;q=0.9,en-US;q=0.8,en;q=0.7'],
            ['de', 'de,en-US;q=0.7,en;q=0.3'],
            ['pt-br', 'pt-BR,pt;q=0.9,en-US;q=0.8,en;q=0.7,ru;q=0.6'],
            ['hi', 'hi;en-US,en;q=0.8,q=0.6'],
            ['ta', 'ta-IN,ta'],
            ['hi', 'hi-in,hi,en-us,en'],
            ['th', 'th-TH,th;q=0.9,en;q=0.8'],
            ['zh-tw', 'zh-TW,zh;q=0.9,en-US;q=0.8,en;q=0.7'],
            ['fa', 'fa,en-US;q=0.9,en;q=0.8,fa-IR;q=0.7'],
            ['nl', 'nl-NL,nl;q=0.9,en-US;q=0.8,en;q=0.7'],
            ['it', 'it-IT,it;q=0.9,en-US;q=0.8,en;q=0.7'],

I noticed the method description for Common::extractLanguageCodeFromBrowserLanguage() incorrectly states that it returns the language and region string. Could this method perhaps be deprecated or a note added suggesting that extractLanguageAndRegionCodeFromBrowserLanguage() should normally be used instead?

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• n/a Wording review done
• Code review done
• Tests updated Tests were added if useful/possible
• none Reviewed for breaking changes
• n/a Developer changelog updated if needed
• n/a Documentation added if needed
• n/a Existing documentation updated if needed