This appears to be a i18n bug. The default required message in a DataView should be a system string and automatically change based on the locale settings of the session.
I will file a bug report for this. Could you perhaps paste the Dutch string you are seeing in a comment (to be sure)?
Error message Dit veld moet verplicht ingevuld worden.
Required messages are not system texts and need to be configured manually. This means that for every attribute that has a required validation rule, you need to specify the error message. You also need to specify the required message for form fields for which you set the 'Required' property to 'True'.
If you do not specify a message, the message will be displayed in the default language, which in your case seems to be Dutch. You can configure language settings, such as which languages are available and which language is the default language, in the Internationalization > Languages dialog in the Modeler.