First check if required language is not in a installed

• Log on to the knowledgebase
• Go the Admin Area
• Click on the Settings menu and choose the Admin tab
• See Langauage setting for available languages
• Choose requred language   

If you can't find the desired language on the list, see how to install it below.
Language packs are available from the KBPublisher download area

• Download the language pack you desire
• Unpack it (The top-level folder of the language pack is named for the language, e.g. 'en' for English, 'de' for German)
• Copy the unpacked files to the [kbp_dir]/admin/lang directory of your knowledgebase
• Follow the steps above to set up language

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This functionality works in version 4.0 and above