To provide a dictionary file for both English and Spanish you will need to include both the en-GB.dic file (or en-US.dic file depending on whether you are in UK or US) and the es-ES.dic files in your dictionary folder. That's the easy bit done.
The hard bit is letting NetSpell know which dictionary file you want it to use. The mere presence of an extra dictionary file will not have any impact on the user interface or NetSpell's processing.
NetSpell uses the value stored in System.Threading.Thread.CurrentThread.CurrentCulture to determine which language file to use for spell checking. By default, System.Threading.Thread.CurrentThread.CurrentCulture will be set to the culture of the machine on which your application is hosted, eg "en-GB". To use a Spanish dictionary file instead, your goal is to change this value to "es-ES" just before you begin spell checking, i.e. ideally it should be set near the top of the Page_Init in your SpellCheck.aspx.vb file.
There are many different ways to achieve this. Here is what I did.
I created a session variable in my main application called Session("Language") and I set the value of Session("Language") to either "en-GB" or "es-ES" depending on which language I wanted to use for spell checking.
Then, in the Page_Init of SpellCheck.aspx.vb I added this code,
If Not IsNothing(Session("Language")) Then
Dim SpellCheckLanguage As String = Session("Language")
If System.Threading.Thread.CurrentThread.CurrentCulture.Name <> SpellCheckLanguage Then
Dim SpellCheckCulture As New System.Globalization.CultureInfo(SpellCheckLanguage)
System.Threading.Thread.CurrentThread.CurrentCulture = SpellCheckCulture
End If
End If
And that's it. Until the spell checking window is closed you are now using the alternative dictionary file.
In your question, you also wanted the user to select a language for spell checking via the user interface. How you do this will be specific to your application. Either by adding a dropdownlist to choose a language and set the Session variable, or some entirely different means. That's really your choice.
Hope this helps.