Multi Language Project

Hello dear community.

I have started a new, private project and am now at the point where I just do not know how to continue.

It's about translations.
I have meticulously ensured that everything in my project is written in English (even if that is not my mother tongue).

The standard translations from the libraries are not a problem, which I got translated quickly. Translating the other texts is not a problem either, just how I can then integrate them in the project, so I'm having my difficulties.

I do not want to have a ready-made solution presented here, but I could use one or the other approach.

I intend to have a drop-down menu in the header bar where you can choose the language.

Since there is also a login area, I want the users to be able to set their language in the profile, which is then automatically called up with the login.

The default language is English, but how can I change it dynamically?

I know well that I also need language files that I fill with keywords and deposit corresponding texts.

About suggestions, I would be very grateful.

And besides: Happy Easter

Your Kighlander

PS: Translated with Google