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Thank you for the reply, 302 is also what we decided after many hours of reading.
The main reason for auto redirect is that we feel it's a better user experience as user will arrive the content target to his/her browser language.

We need to establish if 301 or 302 response code is to be used for our auto redirects based on Accept-Language header.

https://domain.com
30x > https://domain.com/en
30x > https://domain.com/ru
30x > https://domain.com/de

The site architecture is set up with proper inline HREFLANG.
We have read different opinions about this, Ahrefs says 302 is the correct one:
https://ahrefs.com/blog/301-vs-302-redirects/
302 redirect:
"You want to redirect users to the right version of the site for them (based on location/language)."

You could argue that the root redirect is never permanent as it varies based on user language settings (302)
On the other hand, the lang specific redirects are permanent per language:

IF Accept-Language header = en
https://domain.com > 301 > https://domain.com/en
IF Accept-Language header = ru
https://domain.com > 301 > https://domain.com/ru

So each of these is 'permanent'.

So which is the correct?