When Wave transcribes audio and generates a summary, it does so in your Default Language, which is initially set based on your device's language setting.
You can change this setting in Settings -> Default Language
You can change the language of an individual recording by tapping the globe icon on a recording Summary page. When you select a language, Wave will re-transcribe your audio in your selected language. Then a summary is generated based on the translated transcript, in the same language.