We're in the process of localising our website into FR, DE and ES. As part of the current design, all URLs in the website will include the locale e.g. example.com/fr or example.com/es.
Our developer has also enforced this for the default language EN, so all our existing URLs will redirect from example.com/page_1 to example.com/en/page_1
Is this good practice? I'm concerned about the SEO impact.
If your developer is using a 301 Redirect then this is the best practice.
Google guideline (and I guess guideline from other search engines) is simple on this matter. When you are moving a URL and want to maintain its SEO value implement a 301 Redirect when the change is permanent.
Using a 301 redirect for this is indeed the correct way to do.
Should you "move" your EN site from example.com to example.com/EN? Probably not - there's nothing to gain from doing this, it's perfectly fine to have the main language in your main domain (not only that but people probably expect to).
