2566612 - Custom Translation validation failed in Manage Languages

There's one or more of the error messages below when importing a custom locale in the Manage Languages tool:

• The uploaded file has an unsupported locale or no custom translation has been created for it yet.
• The imported file contains unsupported locale or there exists no custom language pack for the locale.
• The uploaded file has a locale that doesn't match the locale you want to customise.
• LANG_PACK_INVALID_COLUMN_HEADER_ERROR_ISSUE:Invalid column headers.
• Issue: Invalid column headers. / Solution: Make sure that the file has two columns; "Key" is the header of the first column and the locale code is that of the second column; and the headers are in the first row.

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SAP SuccessFactors HCM Suite

sf, successfactors, Custom Language, 3 Tier, Tier, Error Language Pack Import, The uploaded file has an unsupported locale or no custom translation has been created for it yet, translation, encoding, invalid column , KBA , LOD-SF-PLT-LAN , Manage Language & Text Replacements , LOD-SF-PLT , Platform Foundational Capabilities , Problem