If you are not adding a new language to SuiteCRM, but rather updating an already installed language, make sure you start by following the appropriate Language Packs Update guide.
Login as English
Go to 'Admin / Module Loader'
Upload and install the new Language pack (zip file)
Click Upload to send the file to the server
Click Install and confirm to apply the changes
Go to 'Admin / Repair / Quick Repair and Rebuild
Login for your new language
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