Almost everything you'll want to do can be done with just HTML/CSS. Some other languages you may come across are:
Java Script: a scripting language that can add extra features to a website.
PHP: a server-side scripting language used to make dynamic webpages where content is stored in a database and the websites 'produced' when needed.
Flash: Not a language but software, again developed by Adobe. Used for, amongst other things, playing music and showing films.
