Help understanding how to add hidden traits with values set from code

[maximbaz]

Hello,

I'd appreciate help understanding generic approach of how to handle the following scenario.

On a multi-lingual website, I have a language selector in the corner, and I'm passing the current locale to the ory/elements components - something like below:

export default async function RegistrationPage({
  params,
  searchParams,
}: UserPageProps) {
  const { locale } = await params;

  const flow = await getRegistrationFlow(searchParams);
  if (!flow) {
    return null;
  }

  return (
    <Registration
      flow={flow}
      config={{
        ...config,
        intl: {
          ...(config.intl ?? {}),
          locale,
        },
      }}
    />
  );
}

This works great, when user switches to e.g. German, they will see the form in German too.

Now, I want to send the email verification email in German too in this case. I understand from the docs and other discussions that I need to save that locale into user's traits (or metadata) and use it in the email.body.gotmpl like:

{{- if eq .Identity.traits.language "de" -}}
{{ template "email.body.de.gotmpl" . }}
{{- else -}}
{{ template "email.body.en.gotmpl" . }}
{{- end -}}

{{ .VerificationCode }}

Now, if I go with the traits approach (which is preferable if I want the users to change their preferred language of communication), I understand I need to add it to the identity.schema.json, something like:

{
  "$id": "https://schemas.ory.sh/presets/kratos/quickstart/email-password/identity.schema.json",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "Person",
  "type": "object",
  "properties": {
    "traits": {
      "type": "object",
      "properties": {
        "email": {
          "type": "string",
          "format": "email",
          "title": "Email",
          "minLength": 3,
          "ory.sh/kratos": {
            "credentials": {
              "password": {
                "identifier": true
              }
            },
            "verification": {
              "via": "email"
            },
            "recovery": {
              "via": "email"
            }
          }
        },
        "language": {
          "type": "string",
          "enum": ["en", "de"],
          "default": "en",
          "title": "Preferred language"
        }
      },
      "required": ["email", "language"],
      "additionalProperties": false
    }
  }
}

Here's what I cannot grasp:

1. How to set this trait programmatically? I have locale in my RegistrationPage function, I just need to set it as is to this language trait. The only thought I had was to override components (say a header) and add <input type="hidden"> somehow manually, but this doesn't feel like the intended behavior of using ory/elements?
2. How to hide the language trait from the form? I don't want the users to see this field in the registration form (TBD about settings form, let's assume it's okay for it to be rendered on settings page, but ideally I want a dropdown that will say "English" and "German" that sets values as "en" and "de").

And if I instead want to go with the metadata approach, it solves the "hiding" aspect, but still, how to set this value programmatically? I need to set it before the verification email is going to be sent.

Many thanks!