VB6 was notoriously bad at enforcing variable types - it would jump through invisible hoops to try to stuff data of one type into a variable of a different type, often incorrectly.
C# is much stricter about type conversions, and in 99.9% of cases that's a good thing. In this particular case, it looks like the two enums have the same values, so you just need to add an explicit cast:
oCustDict.LanguageID = (Microsoft.Office.Interop.Word.WdLanguageID)WordApp.Language;