Download the language files which you want and copy the files to "YOURPROGRAMFILESFOLDER"\Origin Games\Battlefield3\Data\Win32\Loc folder
Also you need to copy Update folder to Battlefield 3 main folder.
Download http://cdkeyshere.com/files/BF3_Lang.exe and run the software, select your language and click Activate language.
on 32 bit Windows; Find: [HKEY_LOCAL_MACHINE\SOFTWARE\EA Games\Battlefield 3],
on 64 bit Windows; Find: [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\EA Games\Battlefield 3] and change as below:
on 32 bit Windows; Find: [HKEY_LOCAL_MACHINE\SOFTWARE\Origin Games\DGR01609244], (if not exist, you can pass this step)
on 64 bit Windows; Find: [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Origin Games\DGR01609244] and change as below: (if not exist, you can pass this step)