@Fougor, you said that your device is already unlocked, so i understand that YOU have unlocked it…
when you unlock a bootloader, that imply a factory-reset (wipe /data : reinitialisation of the device) , that could explain the chinese menu.
if you can access to the fastboot mode, then you can install TWRP or recovery-e instead of the chinese one.
I'll have a look at the recovery-e. I'll have a look on the web.
About global stock ROM: I provided a mirror here: Xiaomi Mi 9T Recovery en chinois Help! - #7 by smu44.
This is the file from XiaomiFirmwareUpdater download link #2.
I try to download the ROM, but the download keeps stopping. I have to restart it again and again.
Then you may have a problem on your side.
For your information, the mirror I provided is in Finland, not China …
I finally managed to install TWRP and change the ROM. It was a bit hard, but the recovery is in English, phew, phew!
So I installed LineageOS.
Thank you all so much for your patience and help.
Happy New Year.
Hello,
You were telling me about LineageOS.
Are you familiar with it? I’m looking for their forum without success.
I guess that on this forum I won’t get any help for LineageOS, will I?
Thank you for your answer.
You might explore from the link to Lineage Discord from https://lineageos.org/community/.
A good resource https://www.reddit.com/r/LineageOS/.
Once you are a bit more accustomed to the device I hope you will return and test Unofficial 
Are you talking about /e/OS Unofficial?
Will this unofficial version be updated regularly?
I’m willing to give it a try.
However, since it’s /e/OS, does this part of the installation ( https://wiki.lineageos.org/devices/davinci/install/variant1/ ) remain the same, where it talks about downloading recovery.img and further down super_empty.img even though it’s not Lineage?
« Installing a custom recovery using fastboot
If so, I’ll do it again this morning.
Yes, I believe, @ronnz98’s ROM, T build, will follow the general shape of the Lineage link which is written for T install.
Be aware, as you very recently installed Lineage that you will likely have a recent Android SPL.
Second edit Not dangerous @Fougor, … just drawing attention to this general /e/OS v1.15 from release notes.
This /e/OS v1.15 includes the Android security patches available until August.
I suggest you ask any follow up questions here: [UNOFFICIAL BUILD] Xiaomi Mi 9T (davinci) for /e/OS-T
Edit I see now why @smu44 was encouraging Global firmware due to
Warning: Before following these instructions please ensure that the device is currently using the firmware version mentioned below.
Is the phone fully working on top of the existing firmware ?
Indeed, my version of Android is version 13.
So going back would be dangerous in what sense?
I’ll go to your site to ask my other questions.
Thanks for all your help.
There may be an anti-rollback protection, preventing you to downgrade (can lead to a bricked device).
For example, downgrading from Android 13 to /e/OS S (Android 12).
I think that /e/OS T is okay, it is Android 13
Also, if needed, you can only install/upgrade firmware: https://xiaomifirmwareupdater.com/firmware/davinci/stable/V12.1.1.0.RFJEUXM/ (just install the zip fil with TWRP).
There may be an anti-rollback protection, preventing you to downgrade (can lead to a bricked device).
For example, downgrading from Android 13 to /e/OS S (Android 12).
Yes, I had read something about this and I understood it.
I think that /e/OS T is okay, it is Android 13
T stands for android 13 and S for android 12 right?
Also, if needed, you can only install/upgrade firmware: https://xiaomifirmwareupdater.com/firmware/davinci/stable/V12.1.1.0.RFJEUXM/ (just install the zip fil with TWRP).
I don’t understand what the firmware update is for. Will it erase all Lineage?
Do I have to do it now? Is it to install /e/Os?
Do you want to give me more explanation, as you say “if necessary”?
Please note I did not use the word dangerous. Just for even more clarity @Fougor by
Settings > About phone > Android version
I mean that you click on Android version in order to read your Android Security Patch Level.
Mea culpa, for me it’s a danger 
Okay, I hadn’t understood.
The vendor’s security patch level is: October 1, 2021.
So this issue I mentioned …
Downgrade of Android SPL is not recommended.
… as I understand it, is Android version and device dependent.
Maybe if you now install /e/OS v1.15-t as I pointed out
This /e/OS v1.15 includes the Android security patches available until August.
… TWRP, possibly, will complain.
To answer, I had not seen the addition, after installing the ROM android everything was fine including after installing Lineage.
On @ronnz98’s post at this address: [UNOFFICIAL BUILD] Xiaomi Mi 9T (davinci) for /e/OS-T - #10 by ronnz98
there’s a new version 1.18-t.
So I can install @ronnz98’s version 1.18-t from his link above and follow the lineage installation procedure?
It seems your LineageOS build is not so very recent anyway (which had been my concern / warning) and from /e/OS release notes https://gitlab.e.foundation/e/os/releases/-/releases we see
This /e/OS v1.18 includes the Android security patches available until November.
So no problem on that score. 
In fact, it’s not as simple as I thought: it’s a matter of versions and patches.
Thanks for clarifying all these points.
I’m going to go ahead and install this unofficial version.
I’ll keep you posted.