Great question, @vincent!
Localize is fully SEO-friendly.
We support alteration of URLs with language parameters, similar to what you described above.
Our solution works perfectly with multiple domains.
Everything is covered – from meta tags, to pre-rendering for the Google Crawler with our Node.js plugin, used primarily by self-hosted websites.
We also have an integration tutorial here. => How to easily translate Webflow websites - Localize Integration Tutorial