Add language support

This needs to be reviewed, but note that this is quite an old patch

I just tested this a bit more on Drupal 9.5.10 and Rest Menu Items 3.0.x and things seem to work fine.
I have noticed though the following though...

Adding a language next to the default language (in this example I use EN as the default language and NL as the second language) does not necessarily mean that /en/api/menu_items/{menu_name} is working. I was getting the same error as mentioned in https://www.drupal.org/project/rest_menu_items/issues/2893872#comment-13... ✨ Add language support Closed: works as designed

{
  "message": "No route found for \"GET \/en\/api\/menu_items\/footer\""
}

The following links were working though
/api/menu_items/{menu_name}
/nl/api/menu_items/{menu_name}

It wasn't until I switched the default language from EN to NL that /en/api/menu_items/{menu_name} started to work.
Obviously because NL was now the default language. Then I switched the default language back to EN and all of the following links started to work.

/api/menu_items/{menu_name}
/en/api/menu_items/{menu_name}
/nl/api/menu_items/{menu_name}

So it seems changing the default language seems to change the endpoints available.

Besided this I am getting aliases returned like this:

EN
https://drupal9.ddev.site/api/menu_items/footer?_format=json

[
{
key: "04e0718c-2c0e-4f2c-a8d8-0b12cf5dee55",
title: "Article #1 EN",
description: null,
uri: "node/1",
alias: "article-1",
external: false,
absolute: "https://drupal9.ddev.site/en/article-1",
relative: "/en/article-1",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "f54a223e-4682-470c-8c59-d70dd54c28c5",
options: [ ]
},
{
key: "d217e9f6-f1d2-4371-8d14-c883f7b0c78e",
title: "Article #2 EN - non translated",
description: null,
uri: "node/2",
alias: "article-2-non-translated",
external: false,
absolute: "https://drupal9.ddev.site/en/article-2-non-translated",
relative: "/en/article-2-non-translated",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "bea04fee-d11a-4818-8a8c-a95f08dc4330",
options: [ ]
},
{
key: "18c38a8b-da0d-448d-834d-fd9eb2de911d",
title: "Front",
description: null,
uri: "",
alias: "",
external: false,
absolute: "https://drupal9.ddev.site/en",
relative: "/en",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: null,
options: [ ]
},
{
key: "c32a9f45-77f1-4112-9cfb-3d077d2d93ea",
title: "Untranslated menu item",
description: null,
uri: "node/1",
alias: "article-1",
external: false,
absolute: "https://drupal9.ddev.site/en/article-1",
relative: "/en/article-1",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "f54a223e-4682-470c-8c59-d70dd54c28c5",
options: [ ]
}
]

NL
https://drupal9.ddev.site/nl/api/menu_items/footer?_format=json

[
{
key: "04e0718c-2c0e-4f2c-a8d8-0b12cf5dee55",
title: "Article #1 NL",
description: null,
uri: "node/1",
alias: "article-1",
external: false,
absolute: "https://drupal9.ddev.site/nl/article-1",
relative: "/nl/article-1",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "f54a223e-4682-470c-8c59-d70dd54c28c5",
options: [ ]
},
{
key: "d217e9f6-f1d2-4371-8d14-c883f7b0c78e",
title: "Article #2 EN - non translated",
description: null,
uri: "node/2",
alias: "node/2",
external: false,
absolute: "https://drupal9.ddev.site/nl/node/2",
relative: "/nl/node/2",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "bea04fee-d11a-4818-8a8c-a95f08dc4330",
options: [ ]
},
{
key: "18c38a8b-da0d-448d-834d-fd9eb2de911d",
title: "Front",
description: null,
uri: "",
alias: "",
external: false,
absolute: "https://drupal9.ddev.site/nl",
relative: "/nl",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: null,
options: [ ]
},
{
key: "c32a9f45-77f1-4112-9cfb-3d077d2d93ea",
title: "Untranslated menu item",
description: null,
uri: "node/1",
alias: "article-1",
external: false,
absolute: "https://drupal9.ddev.site/nl/article-1",
relative: "/nl/article-1",
existing: true,
weight: "0",
expanded: false,
enabled: true,
uuid: "f54a223e-4682-470c-8c59-d70dd54c28c5",
options: [ ]
}
]

I do see however that when a node has no translation the internal path is returned and not the alias of the node of the default language.
I guess we need to decide if that is the responsibility of this module or drupal core?