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Disclaimer: What I describe below is not officially supported and is subject to change without notice! I have not tested these techniques.
The toolbar tooltips come from the file com/vaadin/terminal/gwt/client/ui/richtextarea/VRichTextToolbar$Strings.properties using GWT i18n mechanisms in VRichTextToolbar.Strings. These strings are included in the widgetset at widgetset compilation time.
It might be possible to use GWT localization mechanisms to customize this. See the GWT Developer's Guide for more information. You might need to put your own properties files in a matching directory in your project that is on the widgetset compiler classpath.