Translation layer to be used for menu items and other items

### What is the problem or limitation you are having?

Currently when creating a Toga application, the menu items are only in English with no way to change the language or edit the default menu items easily.

### Describe the solution you'd like

Using a module like gettext and creating locale files for languages. Ideally all items would be translated already for users for any language however that would take a lengthy amount of time. It may also be useful to pick a module that allows dynamic editing/creation of locale files so a dev could add their own translations.

Using a module like gettext would allow for a seamless experience for end users and devs to speed up the language parts of development, especially if we could include a way to add their own translations to their program. Either through toga methods OR the use of a new briefcase entry in the TOML file.

### Describe alternatives you've considered

I am not sure of other translation modules for python, I have only looked into gettext. There may be a better or more well suited option that I am not aware of.

### Additional context

_No response_

The issue does not name any files, tests, or entry points. Begin by locating Toga's menu-item implementation and any existing localization support, then define the translation mechanism, locale ownership, and tests needed to verify translated default and developer-defined menu text.

Written by the indexing model from the issue text.