Append a Lists Contents to another List C#

GlobalStrings.AddRange(localStrings);

Note: You cannot declare the list object using the interface (IList).
Documentation: List<T>.AddRange(IEnumerable<T>).

GlobalStrings.AddRange(localStrings);

That works.

Documentation: List<T>.AddRange(IEnumerable<T>).

Try AddRange-method:

GlobalStrings.AddRange(localStrings);