well, i have experience with the translations in prestashop or wordpress, and less in full custom applications. BUT.. i would advise you to use the () from php, that is if your application uses php, if it uses other languages, look at the documentation to see if it has something thats like this function () or gettext.
more info here :
Back to you: i youl'd use .mo and .po files when dealing with translations, because you can see them as an associative array that has the key = your words and value = the translations text. And i feel you'd have the flexibility to search the web and download already made files and use them in your existing application, if the words are the same.
Well this is my idea. if you guys have any new ones, i'd like to hear them out.