You are using your admin account for administrating Exchange, right? If not – you are on the bad side of life 🙂
So when there is no mailbox available ECP will be shown in the primary language your browser is configured to. If you want to change this just use the correct URL.
For instance https://localhost/ecp/?mkt=EN-us will always use the English locale.
During a migration where your mailbox still resides on Exchange 2007 or 2010 and you want to configure Exchange 2013 settings please use: https://localhost/ecp?ExchClientVer=15
Same will work if you want to use Exchange 2010 ECP: https://localhost/ecp?ExchClientVer=15
Your EXGuru – aka Peter Forster – aka Satschent Peter