I'd say ideally you'd want to focus on English, keeping in mind some languages such as Spanish and Hindi have a large audience on YouTube.
If your English is poor, or your accent is quite thick and hard to understand, it might be better to stick with your language, but focus heavily on English captions.
If English isn't your first language and you want to try doing your videos in English, make sure you speak slower and clearer than you normally would as sometimes accents in a fast pace can be hard to understand.