Our lead frontend dev insists on using <select> as a dropdown for a language switcher in the navbar of a website and then make the page redirect to the localized url using the onChange event with JS.

Am I wrong thinking that's just really, really dumb, or am I just dumb?

I would use an list of links with the flags and change the look using css. Its more userfriendly this way. Yours is possible, but has disadvantages in the usability of normal and people with disadvantages.

I think what's missing is the default detection. The website should check what languages the user has set in the browser and use the highest ranking among these languages that the website supports, or fall back to e.g. English if there is no language preference set.

Also remember to tell Google about that (https://developers.google.com/searc...) to avoid getting downranked in search because of duplicate content.

Too lazy to check right now, but their reasoning might stem from semantics. I don't know how much it would matter for SEO, but it might.