As long as you use UTF-8
as encoding in all parts of your application (Excel, HTML, SQL, Mail Merge). You should be able to edit text in mixed Latin, Arabic, Greek, Cyrillic, etc. at the same time.
Based on your explanation it looks as it will work without any need of modification (at least for the excel->browser->asp->sql). I am not sure how Mail Merge works, you will need to do your research on it further.
Talking about the encoding, it will be no difference if you are using RadTextBox, asp:TextBox or regular Input element in your page.
I hope this helps.
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