I have experienced an issue (Duplicate Record) while enabling language in Dynamics 365 Customer Engagement V9 OnPrem Organization (Vanilla).
The Log file showed error Violation of PRIMARY KEY constraint 'cndx_PrimaryKey_Report'. Cannot insert duplicate key in object 'dbo.ReportBase'
Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=22.214.171.124, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Cannot insert duplicate key exception when executing non-query: System.Data.SqlClient.SqlCommand Exception: System.Data.SqlClient.SqlException (0x80131904): Violation of PRIMARY KEY constraint 'cndx_PrimaryKey_Report'. Cannot insert duplicate key in object 'dbo.ReportBase'. The duplicate key value is (5c4369e7-3c1a-e911-b800-00155d43e6ac, 0, Jan 17 2019 9:47AM).
I tried find the solution into Dynamics community; I identified that its being causing by known issues in Dynamics 365 CE V 126.96.36.19934 and will be resolved in upcoming Microsoft release.
After digging further into forum details i found a resolution in CRM Patch "CRM9.0-Server-KB4458112-ENU-Amd64", (not officially announced yet).
After applying the KB4458112 (V 188.8.131.5241), I was able to enable the language and it worked fine.
I will recommend to install the patch on Dev/Test Dynamics 365 Onprem instance before the production deployment.
Thumbs up if it works for you.
Happy Dynamics 365 CE!!!!