To reproduce: 
1. Add a RadTreeView and populate it with nodes at design time.
2. Use the following code snippet:

Me.RadTreeView1.AllowDefaultContextMenu = True
Me.RadTreeView1.RightToLeft = Windows.Forms.RightToLeft.Yes

When you run the application and right click over a node, the context menu is not in RTL mode.

Workaround: use the ContextMenuOpening event and set the TreeViewDefaultContextMenu.RightToLeft property to Yes.

Private Sub RadTreeView1_ContextMenuOpening(sender As Object, e As TreeViewContextMenuOpeningEventArgs) _
    Handles RadTreeView1.ContextMenuOpening
    Dim defaultMenu As TreeViewDefaultContextMenu = TryCast(e.Menu, TreeViewDefaultContextMenu)
    If defaultMenu IsNot Nothing Then
        defaultMenu.DropDown.RightToLeft = Windows.Forms.RightToLeft.Yes
    End If
End Sub