Add multilanguage support to application

I want a feature to have a drop down with several languages. The default will be English, but when someone changes to Espanol, it will change the text in the application to Spanish. (It will do the translations, you just have to add the functionality). I think it should store this setting in a text file, so it remembers the user's language when the log in, in the future. So I will assign each text item a variable, and that variable will correlate to table (preferably in text) with the text in several languages. Then based on the user's selection for the drop down, it will display the text in that language. Ex. Error1 will be "The data you've entered is invalid." for English, and if the user selected Spanish, it will say (for now, I'll add the translations later): "ES: The data you've entered is invalid." The string "ES: The data you've entered is invalid." should be in a file, so I can change it later. The code is in Python and JavaScript(JQuery).