Hi Peter,
Thanks for using Syncfusion products.
We can localize the customization dialog string by return the localized strings corresponding to the string identifiers with the help of ILocalizationProvider. We have prepared the sample to customize the XP Menus customization dialog in German language for your reference and it can be downloaded from the below link .
Output
Please let us if need further assistance on this.
Regards,
Venkateshwaran V.R.