Change SharePoint Online Site language programmatically on user action

[anisiaalexandra]

Hi great community,

I am writting this post because we need help solving issue in a multilingual environment.
We are busy with migrating sites with customization from SharePoint 2013 to SharePoint Online.
Among the developments we had in SharePoint 2013 we have an application that allowed users to switch UI language between EN/FR/NL using a dropdown.
This was done using an HTTPModule and a custom cookie that was being set on language change. The dropdown for changing the language was installed using SuiteLinkBarDelegate.
We have tried to create an extension that would allow the user to change the language but it seems more complicated than expected. We have tried manipulating the cookies in the site but this does not work.
Does anyone have experience with this or can suggest anything else for this other than asking the user to change the language eighter from profile or browser?
Anyone can explain what goes on behind the scenes (cookie/local storage/session -wise and not the simple theory about which language is picked for the user) when user language is installed? I cannot image that on each call made in SharePoint the language is fetch without using some sort of caching.

Thank you in advance for all your ideas, suggestions and help.