Absolutely any editor implemented with ASP.NET will be multilanguage, by definition. The same stands for HTML + JavaScript editor, the same stands for any .NET application.
You don't really need to "drop down list having languages", because you can write in many languages at the same time; you can even combine any languaes and even different writing systems in the same string.
Perhaps you need something different: to present your UI in different languages, such elements as menus, notification areas, etc. But this problem is easily (I repeat: easily, but for a person familiar with all the target languages or a team of such people) is solved using .NET and ASP.NET
globalization and
localization technology:
ASP.NET Globalization and Localization[
^],
Globalization Architecture for ASP.NET[
^].
There is one more aspect of it: spell check and possibly grammar check. It depends on what technology, dictionaries and grammar databases you want to use. Generally, it is a difficule problem, except simple spell check. For ASP.NET, my best advice would be: don't do it at all. All reasonable Web browsers have some embedded spell checkers; just don't prevent browsers from using them. This solution won't be really perfect, but it will save you from hell of difficult work.
I just don't know what else would you possibly need for help. If, by any chance, you just don't know how to create the editor itself, no
Quick Answers could help you before you learn some programming to an adequate level. I hope it wasn't your question, so I focused only on "multilanguage" aspects.
—SA