Dear Vilas Sir
May the blessings of the Triple Gem be with you always !
Yes, all the desanas of Ven. Waharaka Thero (and other Theros in Sri Lanka) are in Sinhala language.
So there is a scarcity of “pure Buddha Dhamma desanas” in English language.
However there are some desanas available on the internet. You might want to listen to this :
https://www.youtube.com/watch?v=_zBgcGQuEhw&list=PLKr-lYdiK6lLeAp6CM69DOo_fDS5kEhXw
MAY ALL BEINGS ACHIEVE NIBBANA !!!