What is the best for seo using primary domain or subdirectory for the main default page in multilangual website?

I have a website the mainly targets Arabic speakers but I want also to target English speakers, I'm using subdirectories but I couldn't decide if I make the primary domain as the Arabic version or should I use subdirectory for both Arabic and English like this: example.com/ar example.com/en and if I did so, what about the main primary domain, will search engines index it even though it's has no content. note: I don't want to use the primary domain as the language landing page selector... please recommend what is the best thing to do... please don't just throw me link...