Say you have a language header zh-CN,zh;q=0.9 and call Common::extractLanguageAndRegionCodeFromBrowserLanguage() then Matomo will say it is an invalid language code when it should detect zh-CN. As a result, the Matomo UI may not be shown in the correct language and on the Cloud they are receiving emails and UI in English when it should be zh-cn.

Similar eg for pt-BR,pt;q=0.9,en-US;q=0.8,en;q=0.7,ru;q=0.6.