The messages are in this file:

https://github.com/OpenRailwayMap/OpenRailwayMap/tree/master/locales/sv_SE/
LC_MESSAGES

You can edit that with any editor, ideally with an editor that supports
gettext translation files. My personal preference is Lokalize.

Thank you for the answer, and also for guiding me and approving my request.

What happens now? Does it take a couple of days before the changes become visible at openrailwaymap.org? Or will the changes be shown first after a formal release has been done?

There recently was a discussion about how those should properly be tagged and
how this is going to be rendered. Look for the thread "Tagging decision:
ebicab, atc, convel, zub123" in the list archive (it started in January 2021).

Ok. Nothing seems to have happened... I think further discussion is needed. I will look it up and respond in that thread later.

/Patrik