Because you learned to speak in infancy and toddlerhood, you probably couldn't write a reflection paper on what it was like to acquire your first language. However, if you have attempted to learn a second language through acquisition techniques, like spending time around speakers of that language or listening to television broadcasts in that language even if you don't know what they are saying, you can write a reflection paper about this process. If not, you can try to language acquisition technique for the first time. Set up an experiment in which you try to acquire a language in a certain amount of time, and report on the results.
Even if you have never tried immersion techniques to learn a language, likely someone you know has. In fact, at the university where you are studying, there are probably a number of non-native English speakers who would be happy to speak with you about their experiences learning English. Interview them, and ask them to reflect on the language acquisition techniques used and whether they found them successful.
Psychologists have long attempted to determine how the brain makes language acquisition possible for children. How is that children don't have to be taught to speak, that they just "pick it up?" There are a number of theories that have addressed this, and most have been proven false. Make language acquisition the topic of your psychology paper, and explore these theories. Discuss which theory (or theories) you find most persuasive and why.
Watching children as they develop their language abilities can be an amazing experience, and you can record that experience in your language acquisition paper to satisfy course requirements. Ask a childcare center or preschool if you can observe children as they interact with each other, and write down what you see. Next, research theories of child language acquisition and compare what you found during your observation with those theories.