Neither RadEditor, nor RadSpell (which is what RadEditor uses) advance automatically through the available dictionaries. The both offer dropdowns that let the end user choose which is the language to validate the input against. You can see that in the demo I linked.
If you would like to see such a feature implemented on the control(s), I would advise that you post your expectations of how it should work and how should such a configuration be exposed in our Ideas and Feedback Portal: http://feedback.telerik.com/project/108. If it accumulates enough public interest and gets approval from our management, it will be implemented.
If you do not actually need multi-language support, but only several dictionaries, you can consider merging them into one via the dictionary configuration tool as it offers the ability to import single words to a tdf file: http://www.telerik.com/support/code-library/dictionary-configuration.
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