If user types in a wrong value, then an error message pops up

I have an array of Languages in: Range("C1:C100")

The user types in a language in cell: "J4" then click NEXT BUTTON (runs a macro)
If what he typed in does not match a language in the array, then an error message pops up saying: "That language is not supported. Please correct the language to continue."

Here's the code where the error message needs to be added:

Sub Step4()
    Dim i As Long
    Dim strAddress As String
    For i = 1 To 2
        strAddress = Choose(i, "G4", "J4")
        If Len(Trim(Range(strAddress).Value)) = 0 Then
            Range(strAddress).Select
            MsgBox "Please enter " & Choose(i, "the quantity of your order.", "the language of your order.")
            Exit Sub
        End If
    Next i
    Range("J4") = StrConv(Range("J4"), vbProperCase)
    Sheets("StatusAnnual").Range("Z1:AQ7").ClearContents
    Sheets("Image Catalog").Activate
    Range("DI11:DI25").Copy
    Range("CH47").Select
        Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False
    Range("A87").Select
End Sub

Thanks!