.Site.Language.Lang not working as expected with 'where' function

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1

Hi,
I got an issue recently and I can’t resolve it on my own.
In my code $locale returns current language - en

{{ $locale := .Site.Language.Lang }}

following that, I checked that outputs of following expressions

{{ (index .Site.Data $locale).nav.sidebar.navItems }}

and

{{ (index .Site.Data "en").nav.sidebar.navItems }}

return correctly the same data.
sidebar.json file looks like this

{
  "navItems": [
    {
      "id": 1,
      "title": "Parent",
      "url": "/",
      "parent": null
    },
    {
      "id": 2,
      "title": "Child",
      "url": "/",
      "parent": {
        "id": 1
      }
    },
...
  ]
}

In code below

{{ $menus := (index .Site.Data $locale).nav.sidebar.navItems }}
{{ $mainMenus := where $menus "parent" nil }}

I get error error calling where: can’t iterate over
But when I change it for

{{ $menus := (index .Site.Data "en").nav.sidebar.navItems }}

Result is as expected.
I don’t want to have languages hardcoded due to other languages I will use in my app.