The context menu is localized and the strings in it are included in the resource files. Such a file with all strings that the UI of RadRichTextBox uses can be found here. The strings in the context menu normally start with "Documents_ContextMenu".
When it comes to the SpellChecker, there is a compiled dictionary only for en-US culture. More information on creating and using custom dictionaries can be found in this article.
All the best,
the Telerik team