For plesk version 10 and above
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1. Log into plesk control panel.
2. Select "Profile & Preferences" option from "My Profile" section.
3. Select desired language from "Administrator's interface language" drop down menu.
4. Click "OK" button.
For plesk version 9
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1. Log into plesk control panel.
2. Select "Settings" from "Main menu".
3. Select "Interface Preferences" from "Administrator Account" section.
4. Select desired language from "Administrator's interface language" drop down menu.
5. Click "OK" button.