I currently have a Next.Js website where I set the default language by the user one. This means that Google bot reads it as an English website (I have English and French).
I plan to change it: set the main urls as French, and create /en/... paths for the English ones.
Will it impact my seo as it is already indexed on specific keywords? If so, what would be the best practices to put in place to achieve this (on search console or anything else)?
The traffic going to your English pages will have to reindex. I redid my personal website's Web structure last year and look what happened to my site:
It basically killed the traffic off for a half year. I haven't recovered yet.
Basically, you'll lose all your traffic to your English pages and have to rebuild it over the next few years.
Perhaps you could put your French traffic on a subdomain and avoid having to reindex and rerank your pages.
I plan to change it: set the main urls as French, and create /en/... paths for the English ones.
If I correctly understand you, your main language is currently English.
In this case I would highly recommend yout not to change the main language. If you plan to have the website content in two languages, I would recommend, you let the current website structure and the language in the current form. The only thing you'll do: add a folder /fr/ for the french content.
Further, your english and french content versions become their hreflang links. And, if you mean, your certain language version should be counted by Google as main, you markup this version with hreflang x-default.
This solution prevents the case, where your english content is moved anywhere - therefore you avoid its full re-indexing. The only new things Google will get into account, are the new /fr/ content folder, and new hreflang attributes.
