Multi-Language Website, What’s the best approach in terms of SEO?

I have a pretty big website, with a lot of referring domains, 90% of them being natural/whitehat, the current language of the website is English. I would like to tap into other languages as well, but I'm not completely sure which way I should go. What should I choose between using a subdomain or a subdirectory? (ccTLD being out of question because it is expensive and not worth it). I've been reading plenty of articles, but I would also like to know BHWers opinion on this.