As this is posted, I am starting a talk at WordCamp Atlanta, 2019. I am speaking on internationalization (i18n) in WordPress. Below is a link to the slide from my talk as well as additional resources and references that I have found helpful in my own learning and development endeavours.
The slides include code examples with short descriptions. All text should be able to be copy-pasted from the PDF.
Slides (pdf)
The following is a link to a quick plugin I wrote showing different usages of WP translation functions. It also includes a working example of translations in JS files.
Example i18n plugin for WordPress (GitHub)
Resources & References
https://translate.wordpress.org
https://developer.wordpress.org/plugins/internationalization/
https://developer.wordpress.org/themes/functionality/internationalization/
https://wordpress.org/gutenberg/handbook/designers-developers/developers/internationalization/
https://wordpress.org/gutenberg/handbook/designers-developers/developers/packages/packages-i18n/
https://codex.wordpress.org/I18n_for_WordPress_Developers#Using_the_i18n_tools *
http://ottopress.com/2013/language-packs-101-prepwork/ *
http://ottopress.com/2012/internationalization-youre-probably-doing-it-wrong/ *
http://ottopress.com/2012/more-internationalization-fun/ *
https://markjaquith.wordpress.com/2011/10/06/translating-wordpress-plugins-and-themes-dont-get-clever/
https://github.com/timelsass/wp-textdomain#readme
* Page may contain some outdated information