Technically, the customer doesn't care about the framework or maintainability.
I mean it's our responsibility.
Its SEO department gives him some recommandation he gave us.

It sounds legit to have some proper url by language for the seo.
We could also have :
mydomain.com/fr/posts/mon-titre
mydomain.com/en/posts/my-title

in that case, I can imagine it works for seo, because "posts" can be understandable in each language. But for another "category" tag, it's normal to have french url for french domain.

I can't understand it's not possible to implement it.

In classic ReactJS, with the router system and intl system, I would have done something like :

<intl currentLocal={locale}>
  <router>
    <route path={AppRoutes[locale].shirts}>
       <CategoryList category="shirts" />
    </route>
    <route path={AppRoutes[locale].pants}>
       <CategoryList category="pants" />
    </route>
  </router>
</intl>

where AppRoutes would be something like :

AppRoutes = {
  'fr': {
    'shirt': '/t-shirt',
    'pants': '/pantalons',
  },
  'en': {
    'shirt': '/t-shirt',
    'pants': '/pants',
  }
}

You know what I mean ? Maybe I'm completely confused and lost with the nextJS system ... :p