Ah, this is clearer.

This is actually possible, too, in two ways.

The first is to enable assistant languages in the preferences. If a message or a translation unit of a page is translated to the assistant language, this translation will be shown in the sidebar. (I know that @Nikerabbit even thought about some improvements to the UI of this, but it has been possible for a long time.)

The other is to use Special:PageLanguage to set the source language of a translatable page. It's a relatively new and feature, and it's not as well-known as it deserves to be, but I know it has been used for a bunch of pages on Meta. See https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration and search for "Changing the source language".

Does this address your questions?

In T268274#6639207, @Amire80 wrote:

Ah, this is clearer.

This is actually possible, too, in two ways.

The first is to enable assistant languages in the preferences. If a message or a translation unit of a page is translated to the assistant language, this translation will be shown in the sidebar. (I know that @Nikerabbit even thought about some improvements to the UI of this, but it has been possible for a long time.)

The other is to use Special:PageLanguage to set the source language of a translatable page. It's a relatively new and feature, and it's not as well-known as it deserves to be, but I know it has been used for a bunch of pages on Meta. See https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration and search for "Changing the source language".

Does this address your questions?

Well, to be honest..

Special:PageLanguage: You do not have permission to change the page language, for the following reason: The action you have requested is limited to users in one of the groups: Administrators, Translation administrators.

So that's of no use. So I added some language codes to "Assistant languages" on https://meta.wikimedia.org/wiki/Special:Preferences#mw-prefsection-editing.

That's even more useless! This doesn't change the source language, it changes the translation suggestions. So if I set "nl" and I translate a page to French or whatever, the source language is unchanged but now I get Dutch translation suggestions, meaning translation memory went from being barely useful to actively trying to sabotage translation work. No thanks.

When it says "The source language of this group is English. Please select another language to translate into." I want there to be an option to switch the source language as well. (for that translation and that translation only, not for anyone else)

About six months ago I wrote a proof of concept and even demoed this feature. The code is in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Translate/+/588755 but it's not merge-able.

In T268274#6639367, @AlexisJazz wrote:

Well, to be honest..

Special:PageLanguage: You do not have permission to change the page language, for the following reason: The action you have requested is limited to users in one of the groups: Administrators, Translation administrators.

So that's of no use.

This is supposed to be done when the original page is actually written in a language other than English. It changes the source language for everyone, not just for the user who did it. It's not a trivial action, and it's not so frequent, so it makes sense that it's only available to people with a translation administrator permission.

So I added some language codes to "Assistant languages" on https://meta.wikimedia.org/wiki/Special:Preferences#mw-prefsection-editing.

That's even more useless! This doesn't change the source language, it changes the translation suggestions.

Indeed, it doesn't change the source language, but it shows the chosen language in addition to the source language and the translation suggestions. It doesn't change them.

So if I set "nl" and I translate a page to French or whatever, the source language is unchanged but now I get Dutch translation suggestions, meaning translation memory went from being barely useful to actively trying to sabotage translation work. No thanks.

I'm not sure how does it sabotage it. It shows both the translation memory and the assistant language that you choose.

When it says "The source language of this group is English. Please select another language to translate into." I want there to be an option to switch the source language as well. (for that translation and that translation only, not for anyone else)

For that translation or for that translator?

In T268274#6639667, @Nikerabbit wrote:

About six months ago I wrote a proof of concept and even demoed this feature. The code is in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Translate/+/588755 but it's not merge-able.

Interesting, keep us posted.

In T268274#6639670, @Amire80 wrote:

In T268274#6639367, @AlexisJazz wrote:

Well, to be honest..

Special:PageLanguage: You do not have permission to change the page language, for the following reason: The action you have requested is limited to users in one of the groups: Administrators, Translation administrators.

So that's of no use.

This is supposed to be done when the original page is actually written in a language other than English. It changes the source language for everyone, not just for the user who did it. It's not a trivial action, and it's not so frequent, so it makes sense that it's only available to people with a translation administrator permission.

This would only make sense in some scenarios. It could be used if the original is in Korean by changing the source language to English, but even in that case it's kinda wrong. (particularly wrong for attribution purposes)

So I added some language codes to "Assistant languages" on https://meta.wikimedia.org/wiki/Special:Preferences#mw-prefsection-editing.

That's even more useless! This doesn't change the source language, it changes the translation suggestions.

Indeed, it doesn't change the source language, but it shows the chosen language in addition to the source language and the translation suggestions. It doesn't change them.

I guess when I tried it I accidentally picked a message for which translation memory doesn't work. (T101236) So I only saw the "In other languages" section. This could help in some cases, but as (1) translation memory often takes forever to load and (2) this can't be set per translation (unless you change your preferences every time) and (3) the desired source now appears along with the suggestions instead of where the source text is, this isn't really what I mean.

So if I set "nl" and I translate a page to French or whatever, the source language is unchanged but now I get Dutch translation suggestions, meaning translation memory went from being barely useful to actively trying to sabotage translation work. No thanks.

I'm not sure how does it sabotage it. It shows both the translation memory and the assistant language that you choose.

Unless the translation memory fails (as it often does) and you only see the "In other languages" section which looks otherwise identical, so it's very easy to be mistaken for a translation memory suggestion.

When it says "The source language of this group is English. Please select another language to translate into." I want there to be an option to switch the source language as well. (for that translation and that translation only, not for anyone else)

For that translation or for that translator?

Both! Only for that translation (not for anything else that translator translates) and only for that translator. So one user may start translating nl to fy but another could finish it with en to fy.