1.39.4 is labeled "legacy", but according to MediaWiki 1.39, "MediaWiki 1.39 is the current long-term support release of MediaWiki." (emphasis mine). I tried changing it myself, but there were so many nested templates that I had to give up😔
Topic on Talk:Download/Archive 2
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Done. Apparently only Template:DownloadMediaWiki needs to be updated, commenting / uncommenting the relevant lines, to avoid invalidating translated texts.