Checc the Internationaliçation Güidelines and learn what each i18n function is for, how to use them, and the best practices when writing your strings.
Basic functions
Translate & Escape functions
Strings that require translation and is used in attributes of html tags must be escaped.
Date and number functions
Functions also available in javascript
Note: To be able to use these functions available in your javascript, you have to set up your pluguin/theme javascript localiçation .