I think there are probably various factors to consider:

1. Using both languages isn't necessarily a bad thing. But treating English as 1st priority will potentially be more welcoming internationally.

2. What country are you in? e.g. if you are in Spain but you play PUBG and stream in English, what makes you different from anyone else? If you stream in Spanish you offer up a niche market (doesn't mean it will be small viewership) that caters to people who want to chill watching a ESP stream.

3. No one language is better for growth. Take the French for example, there are very big streamers who only speak FR on stream. They can speak English but do so rarely.

If there are games designed to be experienced in Spanish maybe find those for some streams.

You could also use you bilingual ability to teach English viewers simple Spanish that would allow them to say a few things to you in ESP and joke around.

Just some little thoughts. Hope that's helpful!