The SetLanguagePage files in the \Language folder are replaced by new versions from the ProjectTemplates folder.
During migration, existing SetLanguagePage files are replaced with new SetLanguagePage files. These files will be replaced in your application with newer versions from the Project Templates folder.
…\Language\SetLanguagePage.ascx
…\Language\SetLanguagePage.ascx.cs (vb)
…\Language\SetLanguagePage.aspx
…\Language\SetLanguagePage.xml
Customizations you have made to these files will not be retained. If you have customized these files, then you must re-apply your customizations to the new files.
After migration, re-apply any customizations you may have made to the original SetLanguagePage files to the new SetLanguagePage files in your application.
Version 6.0 Application Migration