When adding the Content Search Web Part to a site for the first time, the web part took a while to load, then displayed the error:
One or more of the following resource files failed to load: /_catalogs/masterpage/display templates/language files/en-GB/customstrings.js?ctag=322$$15.0.4569.1000
To look for the missing file I went to Site Settings, then Master page and page layouts, which is under the heading Web Designer Galleries. On that page I navigated through Display Templates -> Language Files. I could see that for some reason the en-GB folder was missing.
The fact that the folder was missing was odd because I’d used the Content Search Web Part before within the Site Collection. Anyway, to resolve the error I created the en-gb folder and copied customstrings.js from the en-us folder and pasted it into en-gb.