This combat footage was retreived from the helmet-mounted camera of one of the Ukrainian 'Azov' batallion members killed in fight with rebels near Yasinovataya. English language can be clearly heard, indicating presence of foreign mercenaries:
https://www.youtube.com/watch?v=TG_oX-TtXRMThere has been lots of news about Azov, being an independent all volunteer ultra-nationalist group (which keeps claiming they are not racist or antisemite), attracting neo-Nazi followers/volunteers from places like Belgium, and even Sweden. English would be a common "international" language for them to use. So this isn't really surprising.