How to send translation arguments to the translation (custom integration)?

If I’d like to translate the sensor status to languages. I refer to the documentation
But I can’t figure out how to define the translation argument.
I set the sensor state:

self.__state = "in_days"

And in .translations, I have a file sensor.en.json with the content:

    "state": {
        "in_days": "in {days} days"

And the sensor state is Translation Error: The intl string context variable 'days' was not provided to the string 'in {days} days - obviously, as I did not define days anywhere. But how to define it?
I read the documentation 10x, searched the source codes, tried to ask on discord. But I am stuck.

I am also having a similar error

Translation Error: The intl string context variable ‘Level’ was not provided to the string ‘Detalhes do log ({Level})’

Mon Nov 18 2019 21:59:39 GMT-0300 (Horário Padrão de Brasília)

Request for failed: 404

I know this is an old thread, but I stumbled on it when I was trying to fix the same problem, and I found the solution. The Developer Docs are trying to explain how to do it, but there are a few things missing in that doc, so let me see if I can make it clear.

Let us say you want to localize a string in a sensor. In the example here I have sensor called sensor.pressure_trend and it can have a state of steady, falling or rising. I now want to display another string in the UI, based on what language the user has chosen.

  • In your custom integration, start by adding a translations directory, if it does not already exist.

  • Now add a file called sensor.YOUR_LANGUAGE_CODE.json example: sensor.en.json.

  • Edit the file and add this

      "state": {
          "trend": {
              "falling": "Falling",
              "rising": "Rising",
              "steady": "Steady"

    Note the word trend right under *state. We will use this later to identify this specific section in the translations. (It could be any word, just something unique)

  • Now in your file you must add a few things before this will work:

  • Import this at the top of your file:

    from homeassistant.helpers.typing import StateType
  • Then when you define your sensor, you must make sure to add a Device Class called “trend” - The same name you used in the json file.
    If you use the SensorEntityDescription this could look like this for the Pressure Trend sensor:

          name="Pressure Trend",
  • The only thing you need to do now, is just to add the StateType to the native_value property, like this:

        def native_value(self) -> StateType:
          """Return the state of the sensor."""

And that is it. If you run the program now it will translate the state of your sensor to what is on the right side of the colon in the json file. And of course you can then add more files like sensor.da.json for danish, etc.

And you can also add more sections in the json file, with more translations for other sensors. Just remember that the sensor must have a device_class equal to the section.

Feel free to poke around in one of my custom integrations, which has implemented this

Hope this helps someone looking for the same answer like I did.