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.
You are 100% correct. Just like you, customers in other countries want to understand what they're buying. Studies have shown they're actually 6 times more likely to buy when a website is available in their native language, and that 85% of global shoppers will not buy at all in a non-native language. Having a translated website also helps people find you/improves your SEO, as even if someone is willing to check out a site in English, they search in their own language.
I'm happy to help further. Just let me know, and thank you!
The best way is that you validate the search volume for the relevant queries in the different languages that you're considering to enable a new version for. By doing a keyword research you'll identify how many people search for your services / products / topics in Spanish from Spain or in English from the US and if it compensate to enable a new version to target them specifically. I have shared the process to follow here: https://blog.kissmetrics.com/how-to-international-seo/.
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.