what version of ReportServer are you using (the default should be English). In any case, you can change the default via the configuration file /fileserver/etc/main/localization.cf. Note that the currently selected locale should also be stored in a cookie, thus, you might need to delete cookies to verify the change. Some further information is also available here: https://reportserver.net/en/guides/config/chapters/UI-Customization/.
on the terminal after changing the config file (sorry, should have mentioned that). You can open the terminal via CTRL+ALT+T. There should also not be any need of changing the ReportServer.html file. What language does it have by default?