[FEATURE]: Possibility to make email text and footer language configurable

Is your feature request related to a problem? Please describe.

First of all - thank you for a great product! It helps greatly in the process!

We are a German company and have multiple domestic and international customers. Our Nextcloud (and our users) have their language set to German. When we want to start a signing process with an international customer, they receive the notification emails in German. Also, the footer text for validation is in German (which is the bigger problem than the German emails).

Describe the solution you'd like

It would be great, if the LibreSign language could be made independent from the Nextcloud or user language. This could be a default language dropdown in the admin settings.
In a perfect solution, a user can configure the whole signing process for a certain document with a certain language. This way, the user could switch to the local language for domestic contacts and e.g., English, for international contacts. As there are already translations for many languages, they should "just" (please note that I understand that this is possibly not that easy) be made accessible via the UI.

Describe alternatives you've considered

I have seen https://github.com/LibreSign/libresign/issues/189, but sometimes the automatic solution might not be the best for a certain document process.
I have tried to switch the user language for the signature requester but this only switched the language for the initial email. The footer text was still in German.

Additional context

No response

No implementation files or tests are identified in the issue. Start by locating the code that selects languages for notification emails and the validation footer, then determine how admin defaults and per-document language choices should interact. Done means both email and footer text can use the selected language without depending on the Nextcloud or user language.