By default sitecore content editor languge for Shell website(Sitecore CMS) is "en". This can be modified globally or per user basis.
Below is the patch config that can be used to change the content editor language to "en-GB"
<site name="shell"> <patch:attribute name="contentLanguage">en-GB</patch:attribute> </site>
This can be done per user basis in sitecore by using User Manager. On each user there is a setting called Language Settings where you can set "Default Content-Language"