Format > Paragraphs > Designer > pick one of your tags, say Body
Properties: Default Font
Language: [ make sure it's US English and not UK English ]
If it is UKE, click in the margin (so no para type is selected), and in that same menu, with everything now set to As-Is or blank, select:
Language: [ US English ]
[ Update All ]
<*> Default Font Properties Only
<*> All Paragraphs and Catalog Entries
[ Update ]
It says US English, but "centre" is accepted, and "center" is flagged as an error. I did the click outside everything ans selected US English. It says "no insertion point" but I continued and updated all. Still "center" is flagged as an error.
Surely the above link would do all of the paragraph formats at once. But my main question is there. I can't find any dictionaries to import.
The dictionaries are located in a Lingusitics folder common to Adobe installations. The exact location varies depending upon the O/S. However, you do not/cannot import any of these files - FM automatically uses the specified dictionary of the paratag or character tag containing the word in question. This allows you to mix languages in a single document.
You can tell FM to Learn a word for your Personal dictionary though via the SpellChecker.
FWIW, in my FM10 installations, both spellings are allowed with US English, whereas the UK English flags the "center" spelling.
Note: the message from FM about "no inserion point" indicates that you didn't do the global update correctly.
The Linguistics files can be found at any of these locations depending upon your operating system and installation location (typically drive C:, but can vary):
\Program Files\Common Files\Adobe\Linguistics
\Documents and Settings\username\Application Data\Adobe\Linguistics
Please note, that messing with the files in these folders will cause FM to crash on start up.
If you select the Help from within FM, it will take you to the correct online version for your installation.
FWIW, Adobe have been less than stellar in updating their documentation, so there are many omissions and older contnt in the newer releases. Adding comments to the online help pages where you have confusion or issues with that content will speed up the corrections/updates.