TL;DR Scroll to the bottom of this post - it works (maybe all along?).
I had installed LibreOffice with danish UI, a while back - just checked but had forgotten about that. So I don’t think it is a bug.
apt list --installed "libobasis24.2*"
...
libobasis24.2-core/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-da-help/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-da/now 24.2.3.2-2 amd64 [installed,local]
libobasis24.2-draw/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-en-us-help/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-en-us/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-beanshell-script-provider/now 24.2.5.2-2 amd64 [installed,local]
...
After struggling a bit I got the danish versions removed:
sudo dpkg --remove --force-remove-reinstreq libreoffice24.2-dict-da
Removing libreoffice24.2-dict-da (24.2.3.2-2) ...
sudo dpkg --remove --force-remove-reinstreq libobasis24.2-da-help
Removing libobasis24.2-da-help (24.2.5.2-2) ...
sudo dpkg --remove --force-remove-reinstreq libobasis24.2-da
Removing libobasis24.2-da (24.2.3.2-2) ...
Apparently libobasis24.2-da-help
24.2.5.2-2 did get installed previously. - Now removed.
I finally got in a state that should work.
apt list --installed “libobasis24.2*”
libobasis24.2-base/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-calc/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-core/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-draw/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-en-us-help/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-en-us/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-beanshell-script-provider/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-javascript-script-provider/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-mediawiki-publisher/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-nlpsolver/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-pdf-import/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-extension-report-builder/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-firebird/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-gnome-integration/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-graphicfilter/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-images/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-impress/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-kde-integration/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-librelogo/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-libreofficekit-data/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-math/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-ogltrans/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-onlineupdate/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-ooofonts/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-ooolinguistic/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-postgresql-sdbc/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-python-script-provider/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-pyuno/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-writer/now 24.2.5.2-2 amd64 [installed,local]
libobasis24.2-xsltfilter/now 24.2.5.2-2 amd64 [installed,local]
But when I open Calc and press F1, the browser still opens and takes me to file:///opt/libreoffice24.2/help/en-US/text/scalc/main0000.html?System=UNIX&DbPAR=CALC&HID=SC_HID_SC_WIN_GRIDWIN#bm_id3155335.
Ahh! It is local! I somehow imagine gtk GUI with HTML and not the browser. Now I’m unsure if this was working all along, since I did not closely inspect the URL.