The 2015 templates support content internationalization by enabling the content provider to identify the language and direction of text elements on a web page. In most cases, language is identified by a two-letter code. In some cases, longer language codes are needed to provide added specificity (e.g., distinguishing between multiple dialects of a given language on the same page). Text direction is identified as either 'ltr' (left-to-right) or 'rtl' (right-to-left).
More information regarding language codes, the authorities that maintain them, and their proper selection can be found in the W3C's article Declaring Language in HTML.
Internationalization vs. translation
Please note the internationalization is not equivalent to translation. The 2015 template do not translate between languages; the content provider is responsible for the proper use, grammar, and spelling of all text, English and non-English alike.
Setting internationalization
There are two situations in which you can set the language and direction of text elements: via datadef form elements and via wysiwyg.