Hi Felix,
Thanks for your interest in Syncfusion products.
We could able to understand your scenario. By default, the record values will be filtered based on the records in the DataSource only. The value to be filtered will be compared to all records in DataSource and then filtered. So, the GridGroupingControl does not have the support to filter the records by ignoring the accents(as per your scenario).
Please let us know if you need any further assistance.
Regards,
Arulpriya