@Charles
You don't need other software to do this - it can be done cleanly enough in Word but there are a variety of methods that can be used depending on what exactly it is that you want to achieve.

Paul's use of IncludeText fields is a robust method that essentially does what Master Document's was meant to achieve. I'm not convinced that numbered lists will avoid running on but apart from that possible wrinkle, this is a clean (and non-macro) way of modularising and single-sourcing the content.

With fields, you can also show/hide content by using document properties and IF fields eg
{If {DocProperty "Language"}="Australian" colour color}
This can work cleanly on a small scale with individual words and it can also be extended to work on large chunks of content but but it will become more and more problematic to edit with large content or non-binary choices.

Because the source document already exists, Paul's described method will likely give the fastest possible workable result.

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Andrew Lockton
Chrysalis Design, Melbourne Australia