How should I approach a 500-word assignment to reflect on and describe my experience with languages?

What you need to say in this essay obviously depends on your own experience with learning one or more languages. 
A good starting point would be to create an introductory paragraph giving an overview of your basic language skills. You would start by specifying your native language. You would then briefly list the other languages you know. You should then include a transition sentence between your introduction and the main body of your essay.
The main body of your essay should include one heading for each language you intend to discuss. Within specific language sections, you might first mention how old you were when you first encountered the language, the way you acquired it, and your level of fluency. You would discuss whether you picked it up informally while traveling or took classes on it. You might also discuss whether you have lived or traveled in a country where the language was spoken and other informal methods of developing fluency such as watching movies or listening to songs in that language. 
Your final paragraph should attempt to reflect on and draw conclusions about your personal experiences with languages.