On the dashboard, there are widgets for "Browser language" and "Language code", respectively.
In custom reports, the dimension "Language" can be used. It would be great if also "Browser language" could be used in custom reports, as "Browser language" makes it easier to analyze usage patters since languages are not divided into sub-groups (such as English - Australia (en-au), English - United States (en-us), English - United Kingdom (en-gb), etc.).
Also, it would be great if dimensions would be named consistently. I.e. if the name "Language code" is used for a widget, then also the dimension in the custom report section should be called "Language code" and not only "Language" as is currently the case.