The first time you ever log into the Exchange 2013 ECP / EAC you are greeted with the language selection screen and time zone, however if you need to change it after that it is tucked away. Below via the ECP I show you how to change the language in exchange 2013 in the ECP this also applies to OWA.
Change Language Settings In OWA
To do this we need to lo-gin to OWA, your URL will be similar to below.
Once logged in, select the cog in the top right hand corner then options.
Then it is just a case of choosing settings from the options menu, then on the right hand menu select regional, you can then edit the language and time zone settings to your requirements. The settings that you change here apply in OWA and also in the ECP / EAC so if you change this for an administrator, when they try to manage Exchange 2013 they will see the language change.