multi-language support

Currently response strings are hardcoded, so creating a project for non-English users requires to carefully search and replace all user facing text.

At least having all text strings centralized in a single location would make this task much easier and less error prone.

Side note: Let me know what is the best way for me to contribute with suggestions like this. Maybe a
[Product Pains page](https://productpains.com/) would be a better option as not to clutter the issues page with these enhancement requests.

The issue names no files, tests, or entry points. Start by locating user-facing response strings throughout the JavaScript, Express, and Mongoose project, then determine the centralization boundary; done means response text has one maintainable source suitable for future non-English projects.

Written by the indexing model from the issue text.