The Windows 8 x64 language packs ISO has the file name of 9200.16384.WIN8_RTM.120725-1247_CLIENT_MULTI-HRM_CL1_X64FRE_MULTI_DV5.ISO.iso, while the Windows Server 2012 x64 language packs ISO has the file name of 9200.16384.WIN8_RTM.120725-1247_X64FRE_SERVER_MULTI-HRM_SL1_X64FRE_MULTI_DV5.iso.
The two x64 language packs ISO images were leaked on Chinese forum PCBeta. The ISO image contains the following languages:
To install the language pack, press Ctrl + R and type LPKSetup.exe. The click on Install display languages, and select the lp.cab file for your desired language. The installation of language pack will commence after agreeing to the license agreement.
Download Windows 8 RTM x64 Language Packs here.