Indication of submission language in form responses

## Opportunity

Following the research sessions on [response](https://app.zenhub.com/workspaces/gcforms-60cb8929764d71000e481cab/issues/zh/364) and [attachment](https://app.zenhub.com/workspaces/gcforms-60cb8929764d71000e481cab/issues/zh/366) downloads through the UI, users shared a need for a clear way to distinguish which form submissions are in English and which are in French. This could help teams quickly identify who is best positioned to handle each submission based on its language.

From client using the API:
> It would be nice if each form submission included whether it was filled in in English or French, i.e. the languages used to view the form at the time of submission. … One use case for this is when client systems want to send bilingual communications back to the user, they can, say, put French text first for French users and put English text first for English users. (I am sure you have seen emails like that, I cannot believe that is a CBSA-only practice.)

A [workshop](https://miro.com/app/board/uXjVJhJpoyo=/?moveToWidget=3458764650900225446&cot=14) with the product team identified a potential solution to this pain point: **add language information to the submission ID**.

## Prioritization
This opportunity was prioritized at a Nice to haves level (Low impact, Low effort) by the product leads

No source files or tests are named. Start by tracing how the form submission ID is generated and exposed to API clients, using the linked response and attachment research plus the workshop proposal as context. Done means submissions indicate whether the form was viewed in English or French.

Written by the indexing model from the issue text.