”Monthly Supporter since December 2018” would be ”Kuukausittainen tukija joulukuusta 2018 lähtien”, where ”joulukuu” is the month name (and the current translation for that string doesn’t correctly account for the inflection either). Finnish has 6 different locational cases alone, so yeah, Finnish inflection can be a pain :)

I’ve only come across this title string (%{taxon} from %{place} in %{month} by %{user}) on an observation page where the date is obscured to month, whereas the (more usual case of) title for non-obscured observations has already been translated to work around the inflection. Although the latter makes for varyingly clunky Finnish, at least it’s technically not incorrect, and hence that’s how the problem is usually worked around in Finnish software translations. I can figure out a similar workaround translation for this string too (that is, take your option b).

I’m still confused by the source string here using %{month}, which then gets turned into %{month_year}, requiring translators to pay attention to the explainer text in the reference, instead of just using %{month_year} in the source directly.