Hi Kristjan Einarsson,
The best option for you is to set Language property of the RadGridView and everything should work fine:
Language="is" stands for Icelandic language. If I'm not correct you can change it with your language.
P.S. By the way RadGridView.CellEditEnded event comes too late in order to change something (edit process is already ended). You could use RadGridView.CellValidating event which exposes editing textbox via EditingElement property.
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