Step 1: Download the language pack for OsCommerce 2.2 or OsCommerce 2.3 depending on your shop version from the ones suggested from.
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https://apps.oscommerce.com/c=languages
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Step 2: Upload and unzip the language package on the ‘language’ folder of OsCommerce CMS
Step 3: Log into your OsCommerce shop admin panel and go to Localization -> Languages and click “Add new language”.
Step 4: Under the “New language” on the right you need to enter the new language with its related data. Also verify the directory name field, it should be the same as in your language pack under \includes\languages\. The Image field should be left unchanged for most language packs.
Step 5: Click on “Insert” and go to the “Online Catalog” or simply open the front page of your site. Now the new language will be added for your shop.