My website mainly targets international users, and I know most people in international countries know English. However, I try to put myself in their shoes, for example, if I lived in Spain I think I would prefer to read Spanish websites over English ones so; this is why I'm thinking adding localization to my website would be a good feature to include for future growth.
That depends a lot on your niche and the type of information that you provide.
For example, for a standard 10-page service website, it's best to have a local version targeting your main locale groups. It gets much more complicated if you run an online magazine - imagine having to translate your 30 news entries a day in 12 more languages.
Also, specific industries and expert-based websites tend to be English-only since the audience is expected to communicate internationally anyway (Clarity is a similar example). If you're looking for a broader audience, then consider translating your content (SEO is also a plus which was outlined above).
As a final note, some countries are skeptical to any foreign languages while others use English as much as their native one.