Delete your account from the Settings Page - scroll down to the bottom of this page and you'll find a link labelled:
"Delete my Readlang account and all associated data"
Click this and confirm that you really want to delete everything.
I'd appreciate if you'd also email me at firstname.lastname@example.org to let me know why Readlang wasn't for you.
IMPORTANT: If you have a currently active premium subscription then you'll have to cancel auto-renewals first, otherwise you won't see the link mentioned above.