It is possible to change the language in Diagrams.net (formerly Draw.io).
The method differs depending if you want the change to be remembered for future visits or only for the current session.
For a permanent change, the instructions are different depending if your browser is accessing the Desktop or Mobile version of the site. Also note that custom language setting is stored in a cookie and if deleted, the language will revert.
Desktop Mode (Permanent)
Under the Extras Menu
, select Language
then English

Mobile Mode (Permanent)
Under the Meatball Menu
, select Settings
then Language
then English

URL Edit (Temporary)
You can alternatively append ?lang=en
to the URL (https://app.diagrams.net/?lang=en
) in both Desktop and Mobile which will override the current language setting for that session. The setting is temporary and you will need to modify the URL each time you visit.