Hi Stefan,
thank you for try.

Try changing the language of the Normal style.
This is the problem... "But I don't see the way how to change it, the menu
under Format button
does not contain Language item as when modifying a paragraph style."

I've used the style inspector and manager and I've found that the language
comes from the table style, so changing the language of whole document,
either in every document or in the Normal template would not work (the table
style would override that).

I'm wondering this is a bug. Not the bad language set, but no UI way to
change the setting.

Cindy does not cover this problem, except the "Note, however, that this will
not affect headers, footers and some other areas that are not included in a
Ctrl+A selection. These will have to be selected and formatted separately."
which brings us back to the problem I have - no way to format it
separately - and "Another possibility is to save in RTF or HTML-Format, then
go into the source code and change/remove the language formatting." which I
particularly did with the docx package itself.

Jan