Hello @Johan den Ouden 

All the Data Grid translations are managed directly by Wijmo Flexgrid Globalization.

For this particular case could you please be more specific about what's wrong with the translation so we can pass that feedback to Mescius support?

Meanwhile, to change this what you can do is add the following (not the best performance but you can try to find a better timing in the component's lifecycle to apply the logic) using a Javascript node on the Grid's initialize event handler:

var myGrid = OutSystems.GridAPI.GridManager.GetGridById($parameters.GridWidgetId).provider;
    
// Check if the FlexGrid instance is valid
if (myGrid) {
    // Attach an event handler to the formatItem event
    myGrid.formatItem.addHandler(function (s, e) {
        wijmo.culture.GroupPanel.dragDrop = "Sleep kolommen hiernaartoe om ze te groeperen.";
        const gps = document.body.querySelectorAll('.wj-control.wj-grouppanel');
        wijmo.Control.getControl(gps[0]).placeholder = wijmo.culture.GroupPanel.dragDrop;
    });
}

//where $parameters.GridWidgetId is the identifier of the Data Grid block

Hope it helps!

Cheers,
GM