The Accept-Language request HTTP header indicates the natural language and
locale that the user prefers when browsing contents. It can be as simple as
Accept-Language: * to accept any languages/locales or as complex as providing
multiple prioritized languages/locales like Accept-Language: es-MX, es;q=0.9, en;q=0.8, *;q=0.5.

In Symfony 5.4, we’ve added new config options to integrate Accept-Language
into your applications seamlessly:

When set_locale_from_accept_language (which is false by default) is set
to true, the request locale is automatically set based on the Accept-Language
value. The value of the `_locale route attribute`_ always overrides this option.

Moreover, if you define the framework.enabled_locales option (which is the
new name of the existing framework.translator.enabled_locales option) the
request locale will be restricted to one of those values (in the above example, it can
only be Danish, Norwegian or Swedish) regardless of the Accept-Language value.

When set_content_language_from_locale (which is false by default) is set
to true, the value of the Content-Language HTTP response header is
automatically set based on the request locale.

These new options will allow your application to better reply to the language
preferences of your visitors and users. It’s already being considered in
Symfony ecosystem applications such as API Platform and Sylius.

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