I was going through the help and read that Multi-Language Add-In cannot handle private UserControls. However, when I scanned my project, it detected the user controls and I could see them under the controls tab. On the other hand, it also showed some errors("Control" not found in Controls-Database) related to user controls under the errors tab.
Please see the attached screenshot for that.
I made some changes into the user controls and those got reflected when I ran the project.
I just want to know what limitations are you talking about as I don't see any problem while working with them.