Google indexing of nonexistent translated URLs

[faforus]

Hello,

I'm managing a website built using Next.js, where each language has its own designated page URL. However, we're encountering an issue with Google indexing nonexistent URLs that are an incorrect mix of language codes and page names. For instance, we're seeing indexed 404 URLs like "website.com/en/kontakt".
Does anyone have a clue what is causing that? Is it related to the library or not?

Existing Pages (examples):

English (EN)
website.com
website.com/contact

Polish (PL)
website.com/pl
website.com/pl/kontakt

Nonexistent Indexed Pages:
website.com/en/kontakt
...and all other available combinations for languageCode/kontakt

We've noticed that Google is indexing over 150 pages such as "website.com/en/kontakt". It's important to note that no link leads to such URL, it can only be accessed manually.

The links are mapped from translations.
t(navbar.${menuItemTxt}_lists, { returnObjects: true }).map()...

{ "slug": "/pl/kontakt", "anchor": "Kontakt", "description": "...", },

<NextLink href={item.slug} passHref> <LinkOverlay> {item.anchor} <Text> {item.description} </Text> </LinkOverlay> </NextLink>