The language of snakes.
Harry opened the secret passage with Parseltongue.
Harry Potter simply said open, but he did so in Parseltongue which is the language of snakes.
parseltongue,
Harry Potter can speak English. Until the Horcrux inside him was destroyed he could also speak parseltongue.
Harry Potter can speak parseltounge since birth because it was given by lord voldemort to him through the scar by mistake. It is also proven in Harry Potter and the Philosopher's Stone when Harry talked to the snake in the Zoo.
He loses his ability to talk Parseltongue.
He speaks in parseltongue. He says "Open" like in Chamber of Secrets.
AnswerYou have to speak parseltongue and then you have to say open and then jump in.
Ron can speak Parseltongue by this time.
Harry can speak Parseltongue, which is snake language. He could do this because part of Lord Voldemort's soul was inside him. Once that piece of soul was destroyed Harry couldn't speak or understand Parseltonuge. It is unknown if he ever learned a foreign language.
English, Parseltongue (fictional language), French (very little)
When Lord Voldemort hit Harry Potter with the killing curse in the Forbidden Forest it killed the part of Voldemort's soul inside Harry. This meant the connection between the two of them were broken so any powers that Harry only had because of Voldemort's soul were gone. This included the ability to speak Parseltongue, it also meant Harry could no longer access Voldemort's mind.