I added a small feature that a competitor was missing, and it caused a 2,000% month-to-month growth in a specific segment of users.
Adding support for Spanish caused my monthly unique spanish-speaking users to increase from 6 to 155.
What made me decide to add this feature - I check up on sites that are similar to Fortnite Dashboard regularly, and earlier this month I saw that one of my competitor's users was asking if they could make their site more accessible to Spanish-speaking users. After seeing that request, I decided to add multilanguage support to my own site.
I was able to refactor my site overnight, and the next morning I replied to the above user's comment with a link to my site, and announced on my social feeds that all content from that week forward would also be available in Spanish (with a disclaimer that I don't actually speak Spanish and am using Google Translate).
Just a few hours after the announcement, I saw a large spike in users from Spain, and since then I've had a steady flow of daily users whose language is "es-es" according to Google Analytics.
At the time I started working on the feature, it really didn't seem to me like it would be very important, but it ended up being pretty valuable as far as gaining new users!
Because of this success, I've started paying more attention to the language tab in google Analytics, and using those numbers to decide which language(s) to add next.