Cross references of any kind actually do use hidden bookmarks which can only occur once in a document. But the basic setup for having language-specific content is not that complicated. Attached is a file showing how I set up a sample with content controls tagged with a language-specific code along with a macro that allows the user to select which language should be shown.
Code:
Sub LocaliseMe()
Dim aCC As ContentControl, sLang As String
sLang = InputBox("What language should be displayed? Your choices are: EN, DE, All", _
"Choose Language Code", "All")
For Each aCC In ActiveDocument.ContentControls
Select Case sLang
Case aCC.Tag
aCC.Range.Font.Hidden = False
Case "All"
aCC.Range.Font.Hidden = False
Case Else
aCC.Range.Font.Hidden = True
End Select
Next aCC
'set options to make ensure hidden text is not displayed on screen
ActiveWindow.View.ShowAll = False
ActiveWindow.View.ShowHiddenText = False
End Sub