Multilingual site - subdomains or subdirectories?

So I'm creating my first "multilingual" website e.g. I'll have the same (or slightly modified) article translated to 15+ languages, then hosting it on the same domain. Normally I'd just create new sites for each language, but this niche has such low competition/search volume it isn't really worth it.

Does anyone have experience with creating sites like this? Would it be better to use subdomains or subdirectories from an SEO perspective?

I'm leaning towards subdirectories myself because it seems like I'd get more benefit from the strength of the domain this way, but I see a lot of other people using subdomains so I figure there might be a reason?

I'll also listen to wordpress plugin recommendations if you have any.

Thanks.

I recommend to use WPML for the purpose on Wordpress and you should use subdirectory, this is the best solution.
Google read it and understand that they are for languages, if you use subdomains, you need to redirect them to a subdirectory, and 15 languages is a load of work to do, and you risk to create duplicate contents, so you need to work on the canonical a lot.

You should read this:
https://support.google.com/webmasters/answer/182192?hl=en&ref_topic=2370587

I would recommend subdomains so that your different language content can clearly be distinguished. You may also be able to geo target via webmaster tools.