My previous version of Parallels Desktop for Mac was English/Ger
Yes, with the release of Parallels Desktop 5.0 for Mac this became possible.
Please note, if you own a copy of Parallels Desktop 3.0 or 4.0 in any localization and would like to upgrade to 5.0 in another localization you can do this only once.
You would need to purchase an Upgrade to 5.0 in the needed localization, install Parallels Desktop 5.0 for Mac for the same localization the 5.0 Upgrade key is and activate with your 5.0 localized key + previous key for Parallels Desktop 3.0 or 4.0 for another localization.
Example: If you have 4.0, English, but would like to upgrade to 5.0, German, please:
1. Order Parallels Desktop 5.0 Upgrade, German from the German webpage:
http://www.parallels.com/de/store/
2. Download the installation package for Parallels Desktop 5.0 for Mac, German at:
http://www.parallels.com/download/
3. Install Parallels Desktop 5.0 for Mac, German and activate using your 5.0 Upgrade, DE key and your 4.0, EN key.