Thanks, but that wasn’t actually the problem (our entire site is already in English).
The problem was that we were hitting our email sending limit on our hosting plan. It allowed for 300 emails to be sent per hour and with thousands of people registering and performing actions on the site that generated notifications it meant we were hitting that limit. So activation emails were only being sent when that limit had not been reached yet that hour.
If anyone else is having this problem, I suggest you speak to your host about how to increase your email sending limit, or look for ways to disable other emails sent by the site.
