I have an online store with over 8k products, and my only experience with translating WP sites was with Polylang. Polylang is fine for smaller websites, but I didn't like how you have to double up your content every time you add a new language. E.g. in Pages in the admin interface there's the page in the "original" language, and then the page in the other language when you use Polylang. Add one more language, and you have 3 copies of the same page clogging up your list.
I get that the plugin will have to add content in the database, I just don't want to see it on the admin page so that managing the website is as easy as possible. Basically, we have a base language, and because I live in a multilingual country, we want to offer a couple more languages. I don't want to have to worry about content in the other languages, basically I just want to submit the translated strings once and then forget about it.
Then ideally I would manage the item from the "original" in the original language, e.g. price, stock availability, etc. with the changes made in the "master" or "original" product, that is the one that appears in the Products list in wp-admin, pushing it to the "child" copies in other languages.
If that makes sense lol.
Since WPML only has a paid version, I thought I'd ask here first. Is it convenient to use for such a huge online store?
Also if I'm able to import translation strings through a CSV that would be cool to save on auto-translate credits, since our suppliers actually do have item names and descriptions in the different languages.
In WPML, only the Orginal product can manage pricing, stock, etc.
I have sites using WPML and WC with no issue. Just takes a bit of time to get use to the flow because there is so many settings.
With the amount of products you have, keep the product pages simple