Translation for Server-Side Messages

• Existing feature enhancement

Messages from server-side operations are not translated
For instance:

https://github.com/processing/p5.js-web-editor/blob/develop/server/controllers/user.controller.js#L60-L61

https://github.com/processing/p5.js-web-editor/blob/develop/server/controllers/user.controller.js#L147

https://github.com/processing/p5.js-web-editor/blob/develop/server/controllers/user.controller.js#L176

Feature enhancement details:

In order to not break the API, maybe introducing an extra field in the response to identify the string to display, and the field that triggers that response, and get the translation in the client, according to the language of the client.

For example , an extract from these lines
https://github.com/processing/p5.js-web-editor/blob/develop/server/controllers/user.controller.js#L102-L108

 if (user) {
      return res.json({
        exists: true,
        message: `This ${checkType} is already taken.`,
        type: checkType
      });
    }

would be

 if (user) {
      return res.json({
        exists: true,
        message: `This ${checkType} is already taken.`,
        messageKey: ServerTranslation.UserTaken,
        type: checkType
      });
    }

Start by reviewing the referenced branches in server/controllers/user.controller.js, especially the responses around lines 60-61, 102-108, 147, and 176. Determine how to expose translation keys without breaking the existing API, then verify that the client can select the corresponding translated message for its language while preserving the current response fields.