As this is posted, I am starting a talk at WordCamp Birmingham, 2016. I am speaking on internationalization (i18n) in WordPress. If I have the time, I plan on publishing a blog post with a summary of my talk and everything that was discussed. In leu of that though, a link to my slides is down below this text as well as a list of resources that I have used in the past to learn what I now know.
Resources & References
https://translate.wordpress.org
https://developer.wordpress.org/plugins/internationalization/
https://developer.wordpress.org/themes/functionality/internationalization/
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/
* Page may contain some outdated information