Hi Henrik Söderlund,
The correct value that you have to provide in your custom localization manager to localize hyperlink tooltip is Documents_RadRichTextBox_HyperlinkToolTipFormatString. Calls with Documents_HyperlinkToolTipFormatString are considered a bug and should be removed in the 2011 Q1 SP1 version.
Please note that the first value is needed for control's initialization (in RadRichTextBox static constructor), so you have to set your custom localization manager before control's creation. If you need to change this value later, you can directly set RadRichTextBox.HyperlinkToolTipFormatString property.
For your convenience, I have attached a resource file with all strings and keys that have to be localized for RadRichTextBox.
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