I've been using Hello Talk for over two years, most people that started a conversation with me, said something along the lines of:
Hello, my name is XX. If it's ok for you, can you teach me english? (ofc you'd switch for japanese here)
But that's more if you are asking for help, in your case sounds like you are rather offering help or wanting to exchange both languages "equally" right?
In that case I would say:
Hello, my name is XX. Let's study english and japanese together? Please accept me as your language partner! (It maybe sounds a bit blunt when you translate to english, but I think it's totally ok in japanese!)
As pointed by Eric, in case you already know the person and just want them to feel free to contact/message you:
Please feel free to contact/message me, when you feel like practicing english.
Please fell free to contact me whenever you feel like practicing english.
Here, I believe the ね softens the "request" while still being polite.