I use RWML from S-4-S in one Foundation website (normally 2 or 3 languages). RWML comprises 9 stacks, with one for improving SEO and another one for translating the menu. The users language choice is tracked by a cookie with a predetermined life, typically 90 days. I cannot see the problem with flags / text to choose language, auto language detection is more problematic as people move. The S-4-S website has much more information.
Yes, indeed, as Phil says, RWML (https://stacks4stacks.com/rwml/) is definitely your best ally. It’s wonderful, it has a learning curve but once you know how to use it, it’s the most effective option you’ll find. Apart from @willwood customer service that is always impeccable