I18n fixes

[eri-trabiccolo]

Description

Various i18n fixes. Fixes #2324 Fixes #2429

Additionally introduces settings to customize the LifterLMS CPT and Taxonomies slugs (see screenshot)

How has this been tested?

manually

Screenshots

Types of changes

bug fix + new feature

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.

[brianhogg]

Closing, new branch made with similar fixes.