When you open a doc in a different language version of Word than was used to create the doc, Word does not translate built-in style names to the language in use.
To work around this problem:

1. Display field codes by pressing ALT+F9.
2. Change the reference to the correct name. For example, change the contents of the STYLEREF field from "Titre 1" to "Heading 1" if you are converting from the French to English.
3. Press F9 to update the field.

Please let me know if that works