Hi Sauny,
I think you want Word Smart Documents bound to XML schemas. Too bad Word lost the lawsuit against thast company in Texas and they had to dumbdown the feature, but take a look at Word - you should be able to do what you describe using a Word Smart Doc.
In InfoPath, you can create structured forms. For a legal document (a contract), you could do a similar thing using dropdowns inserted between hardcoded text. Just type your text and insert a dropdown for the word you want to change. Dropdown is bound to the secondary data source. If you are trying to do something like translation, I recommend trying to find an off the shefl piece of software for that.
Rich text controls cannot support the feature you want, but you could use the taskpane (rich client only) to provide a mechanism for replacing text in a rich text control with a list of pre-baked options. We did a form for a multi-speciality clinic that inserted medication names into a prescription area. Unfortunately, text in a rich text control cannot be bound to a separate schema (it exists as XHTML in the form).
Hope these ideas help a little. Sorry I couldn't be more helpful.