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Many languages are available for your to download and install, directly from the PrestaShop servers. Your PrestaShop install will take care of downloading the language pack, unpacking it and create the correct sub-folder in your installinstallation's /translation folder.
You can also update the currently installed languages, likewise directly from the PrestaShop servers. But be reminded that any change that you might have made to a language pack will be lost if you update it.
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Export a language
Either as of a way of backuping making a backup of your customizations, or of sharing them with other PrestaShop installs, you can create your own language pack using this form.
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