hreflang tag for very unique international website

Hey there!

This is my very first post. Hopefully, this will be helpful for more people than just me.

For a current client, I am facing a very particular situation when it comes to hreflang settings. In this case, the client has a mix of direct and pretty common alternates plus other not so common but with a language in common.

This scenario might require the use of HTML lang just language + hreflang lang to be language-region in my opinion.

Example My client will be selling a "product" and this product will have different possible alternates. This is just an example of course, as way more alternates should appear.

• client.com/es/es/product (html lang = es) (hreflang lang = es-es)
• client.com/en/es/product (html lang = en (hreflang lang = en-es)
• client.com/it/es/product (html lang = it) (hreflang lang = it-es)
• client.com/ar/es/product (html lang = es) (hreflang lang = es-es)
• client.com/de/es/product (html lang = de) (hreflang lang = de-es
• client.com/it/it/product (html lang = it) (hreflang lang = it-it)
• client.com/en/it/product (html lang = en) (hreflang lang = en-it)
• client.com/es/it/product (html lang = es) (hreflang lang = es-it)
• client.com/ar/it/product (html lang = ar) (hreflang lang = ar-it)
• client.com/de/it/product (html lang = de) (hreflang lang = de-it)
• client.com/es/pl/product (html lang = es) (hreflang lang = es-pl)
• client.com/pl/pl/product (html lang = pl) (hreflang lang = pl-pl)
• client.com/es/be/product (html lang = es) (hreflang lang = es-be)
• client.com/nl/be/product (html lang = nl) (hreflang lang = nl-be)
• client.com/es/at/product (html lang = es) (hreflang lang = es-at)
• client.com/de/at/product (html lang = de) (hreflang lang = de-at)

I asked this question in Google Webmaster Forum and as far as I know, my hreflang proposal should be fine, however, I have big doubts when it comes to HTML lang tags. I am aware that Google ignores these tags (HTML lang), but I want to avoid any future issues so I really wonder if this "just language" HTML lang tag + "language+region" combination would work fine as I've found in tools like the Merkle's checker or ahrefs audit that they spot these combinations as wrong.

I am aware that many websites use the same HTML lang + hreflang lang "language+region" combination just as:

https://www.playstation.com/en-us/games/ratchet-and-clank-rift-apart/

What would you think?