I have WPML with English and German. Even though I switch the WordPress language to English, the UpDraft Dashboard is in German. The emails are also sent out in German.
When a plugin has translations, it just says to WordPress, “here are some translations, if you want to use them”. It does not get involved in the decision about which translation to use. Therefore, I think you’ll need to talk to the WPML people to look into this.