Hey! Sorry for a late reply as I have been very busy lately and hardly posted to steemit.
You can choose tags yourself and nothing is forced to be the main tag. By default, SteemPress uses the tags you chose for your WordPress article. But we also added a feature where you can set your own default first tag.

Some users requested this as they predominantly wrote in their native language on their wordpress blog and would therefore like to automatically set the main tag as "deutsch", "spanish" etc.

I hope that answers your question :)