Legally and in accordance with Windows End User License Agreement you have no other options than to upgrade to Windows 7 Ultimate. Language packs are available as optional updates in Ultimate and Enterprise editions
only (Enterprise = Ultimate with volume licencing).
You simply can not install language packs to Home Premium and maintain your system and activation validity. I strongly advice you against trying any third party solutions, reasons can be found in this
post. To put it simply, you have to invest some money in Ultimate to get the additional language pack feature.