Internationalization (“i18n”) is a domain of computer science focused on making software accessible across languages, regions and cultures. A combination of those is called a locale.
On the most abstract level, Gecko internationalization is a set of algorithms, data structures and APIs that aim to enable Gecko to work with all human scripts and languages, both as a UI toolkit and as a web engine.
In order to achieve that, i18n has to hook into many components such as layout, gfx, dom, widget, build, front-end, JS engine and accessibility. It also has to be available across programming languages and frameworks used in the platform and front-end.
Below is a list of articles that introduce the concepts necessary to understand and use Mozilla’s I18n APIs.