Despite being a Mandarin-language production, four languages were spoken during filming. Most of the cast spoke Mandarin (even westerner Andre Morgan, who was already a multi-lingual producer for Golden Harvest), while Hwang In-Shik and Kim Ki-Joo spoke Korean, Kazuma Kenji spoke Japanese, and Jhoon Rhee & Anne Winton spoke English. Despite being South Korean born (and Korean being his primary language), you can tell Rhee's lip movements more closely match lines from the dub and/or the English subtitles. While it's not known for sure why he spoke English instead of Korean during the production, it may have been to assist when the film would receive its eventual US release/dub, where the majority of his students (especially in the Washington DC area) were located.
Bruce Lee was not in the movie "When Taekwondo Strikes," but he played a key role in getting the film made. He convinced Golden Harvest to produce the film, starring his friend Jhoon Rhee, a well-known Taekwondo instructor. Bruce Lee's involvement was primarily behind the scenes, helping to facilitate the production.