internationalized routing should not require overrides for all languages

[murbanowicz]

Is your feature request related to a problem? Please describe.

Some words are international or common for quite some languages.
If the path is the same for almost all locales supported by the app, we should not have to copy paste the same path all over just to be able to override one specific language.

Describe the solution you'd like

The functionality and therefore the typings, should require only partial override if provided instead of all languages supported by the app.

So instead of doing this for specific path in Italian:

    "/foo/[slug]": {
      en: "/foo/[slug]",
      de: "/foo/[slug]",
      es: "/foo/[slug]",
      fr: "/foo/[slug]",
      pl: "/foo/[slug]",
      it: "/mozarella/[slug]",
    },

We should be able to do this:

    "/foo/[slug]": {
      it: "/mozarella/[slug]",
    },

Describe alternatives you've considered

Providing a path for every locale.

[amannn]

I guess this could be considered, yes. Related: #990