This is a cross-project feature suggestion that grew out of conversations that I had with Wikimedia Israel's Michal @Lester, WMF Global Collaboration team meetings, and some other things that I experienced over the years as Wikimedia project editor and translator. It's just a beginning of an idea, which may grow into a larger project over time. As I often do, I am adding a few people who may be interested to the subscribers list; if this doesn't interest you, please remove yourself and accept my apologies.

There are several scenarios of contributing to wiki projects in multiple languages, which are currently sometimes performed by editors using wikis' built-in editing and discussion tools, but aren't addressed by dedicated software. In general, translation of article content is now addressed by Content Translation, but there are more advanced scenarios that are done manually. (In this task, "manually" means "using built-in wiki editing and discussion interfaces without any extra features".)

1. One scenario is improving the source article. Sometimes, when translating, the translators also improve the source article or leave comments about it on the source article's talk page. This is generally desirable, because in the wiki culture it's understood that no page is ever perfect, but not all translators are actually comfortable with doing it: some don't know the source well enough to write it, some don't feel that they belong to that editing community. That's OK—contributors should write wherever they are comfortable. And some translators may be generally comfortable with editing the source wiki, but in Content Translation the source column is not editable, and to edit the source page one needs to open another browser tab (also, in the current iteration of Content Translation this may have the very unfortunate effect of destabilizing the translation process itself).
2. Another scenario, closely related to the previous one, is leaving feedback about the source page. Perhaps the translator doesn't want to edit the source article, but to add some substantial feedback about mistakes, missing sections, or anything else. Of course, it's possible to post it to the source article's talk page, but this may be hard for reasons similar to the above scenario: people may be uncomfortable writing in the source language, they may think that the talk page culture in that wiki is very different, or they may think that their comment may go unnoticed.
3. The next scenario is updating an article with information from an article in another language. Currently, this is currently really covered by Content Translation, which is intended as an article creation tool rather than an article editing tool, but it comes up very frequently as an editors' wish. This is already described in task T90209, but I'm mentioning it here, because it is related to the other scenarios.
4. There should be a way to have central cross-project and cross-language discussions about topics. This is usually OK, because discussions about articles mostly happen between people who speak the same language and are accustomed to editing the same project, but there are many cases in which having cross-project discussions would be very beneficial. Even without talking about different languages, it may often be useful to coordinate some discussions about Charles Dickens in Wikipedia and Wikisource, or some discussions about Yosemite in Wikipedia and Wikivoyage. Currently this can only be done on separate talk pages with frequent manual use of pinging other editors using Echo's mentioning mechanism, because neither talk pages nor watchlists can currently work across projects. Having central talk pages would also help facilitate scenario 2 in this list, leaving feedback about the source page of a translated article. There are no interlanguage links on talk pages, and there is no good reason for that. The closest thing that we currently have to a global talk page about a topic is the talk page of the topic's Wikidata item, but as far as I know it is not used in such a way, and even if it would be used for this, it would require frequent use of pinging. Of course, there's also the problem of having one discussion in different languages. T98728: View translated messages in Flow may be somewhat helpful here, but it will not be a perfect solution because machine translation is always faulty and it's not available for all languages. Nevertheless, it's a thing to consider.

There are definitely more scenarios of this kind. Please add your ideas as comments, and feel free to treat this task as a wiki and use the "Edit task" link to edit the description. As noted, this is just the beginning of a big discussion, and if this ever gets actionable, it will probably be split to many more tasks.

Thanks :)