There is a number of functions available in ClassicPress which can be used in localising code:
__()- this function takes a string and returns the translation if it exists.
_e()- this function takes a string and echos the translation if it exists.
_n()- this function allows plurals to be handled. It takes 4 parameters: the single form, the plural form, the number based on which one or the other will be displayed and the text domain.
_x()- this function is to prevent collisions of similar words. If you plugin uses pair in the sense of a couple of people and pair in the sense of pairing a device via bluetooth as well, collisions could arise. A foreign language might well use very different words for the two.
_ex()- this function is the same as the above, but echos the result rather than returning it.
_nx()- this function is a combination of pairs and plurals.
There are also six functions which allow us to escape the value for safe usage in attributes and HTML. Considering the above functions, the names are very descriptive: