By default OCS only installs the US English language pack for Response Groups. There are 11 other language packs you can install. The RGS uses the UCMA 2 Speech language packs which can either be found on the OCS install CD in the \Setup\amd64\Setup\Speech\ directory. You can also download the packs from here.
After running the msi to install the language pack you need to register the language pack. To install the language packs you need to run the rgslang.exe application from the OCS 2007 R2 Resource Kit. From a cmd prompt run RGSlang sync then restart the RGS service. Full details of the install process can be found on TechNet.
As well as the speech packs you can also manage OCS in different languages or display the OC Tab in a different language by downloading the Response Group Service Language Pack.