MongoDB add dynamic properties in aggregate

Question

I have the following schema structure

[
  {
    "locale": {
      "en": {
        "dashboard": {
          "DASHBOARD_TITLE": "Some title"
        },
        "contact": {
          "CONTACT_TITLE": "Some title"
        }
      },
      "fr": {
        "dashboard": {
          "DASHBOARD_TITLE": "Some title french"
        },
        "contact": {
          "CONTACT_TITLE": "Some title FR"
        }
      },
      "pr": {
        "dashboard": {
          "DASHBOARD_TITLE": "Some title portugues"
        },
        "contact": {
          "CONTACT_TITLE": "Some title PR"
        }
      }
    }
  }
]

I have MongoDB script to fetch the values in given format

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "locale": {
      "fr": {
        "contact": {
          "CONTACT_TITLE": "Some title FR"
        }
      }
    }
  }
]

MongodbScript

function getDataFromMongo(lang, feature) {
await this.staticDataModel.aggregate([
      {
        "$project": {
          locale.fr: { //here fr is the lang param. how to append lang dynamically 
            "$arrayToObject": {
              "$filter": {
                "input": {
                  "$objectToArray": `$locale.${lang}`
                },
                "as": "el",
                "cond": {
                  $eq: [
                    "$$el.k",
                    feature
                  ]
                }
              }
            }
          }
        }
      }
    ]).exec(); 
}

What is the solution for appending dynamic properties? Here locale.${lang} this won't help for me