Multilingual comes with a flexible "Frontend Language Detection Stack" that supports different URL Structures and uses the following methods for frontend language detection:
With each page request Multilingual will execute these methods in chronological order until a valid language-code is detected. A language-code is considered "valid" if it complies with the formal requirements (two letter-code) and matches one of the frontend languages that have been defined in the configuration.
If no method suceeds in returning a valid match the extension will fall back to it's own default language.
If the URL Query string contains the parameter language and this parameter's value is a valid language-code this language-code will be used as frontend language:
domain.com/page/?language=en
If the URL path starts with a valid language-code this language-code will be used as frontend-language:
domain.com/en/page/
If the domain or subdomain is mapped to a valid language-code this language-code will be used as frontend-language:
domain.co.uk/page/
If no valid language can be detected in the URL the extension will search for a cookie named multilingual. If this cookie exists and contains a valid language-code this language-code will be used as frontend-language.
The cookie multilingual will be saved whenever one of the 3 URL-based language-detection-methods detect a valid language-code.
Note: While the availability of a language-cookie offers the possibility to serve multilingual content without a fully-fledged multilingual URL-structure (and rely on the cookie-value once it's set) this is not recommended. The cookie is meant to be used for redirecting to a multilingual URL, not for replacing it.
If the cookie multilingual is not set or doesn't contain a valid language-code the extension will have a look at the browser settings and check if any of the client's accepted languages defined in the http_accept_language-header matches one the frontend languages defined by the extension.
de-DE,de;q=0.8,en;q=0.7,fr;q=0.4
In case of success this method will always return the "best possbile match" as it scans the accepted languages in order of their relevance (defined by the q-parameter in the header-data). In case of the example shown above Multilingual would choose en as frontend language if the language configuration would be set to fr, en.
If none of the 5 methods above return a valid match the extension will fall back to the configured default language.