I wouldn't use fields and bookmarks. I would use Content Controls instead. With those content controls, I would also probably map them to a custom XML file. A solution might involve a library of language/masthead specific XML files which could be substituted into the file to implement the changes.

I would not create three separate userforms - the same userform should do language, masthead and data fields in one action.

As to your Challenges/Limitation/Questions
1) Can building block contains fields/bookmarks? Yes they can but it is easier and cleaner to use mapped content controls
2) We have 120 mastheads per language, can the building block.dotx support that large number of data? Yes they can but if you are going to multiply this by 6 for each language, I would recommend you put each language set in a different template/addin and load the required one only.
3) Since I am calling directly the masthead using in VBA after entering the template code, am I not better using InsertFile instead of building block (this will prevent me asking user to go in their AppData to do changes or to have to load all blocks data in the master template). I don't understand this question, you must be distributing the template(s) to the users somehow already so why would this be a problem?
4) I can't use third party macro such as Graham Mayor addins Bummer dude! Yet you ask third parties for advice on this forum. Does this mean we can't provide you code samples or advice? You are welcome to recreate all this functionality from scratch - it's not my time or money you are spending to do so

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Andrew Lockton
Chrysalis Design, Melbourne Australia