How can I change the newsletter message?

in my website, once someone subscribes, says
“choose your newsletters” to, well, choose your newsletters among the ones that has in it.
How can I change this sentence to another? Well, more specifically, where in ghost theme files can I do such thing?

If it’s in a popover, that language is from Portal, not from the theme.

I wonder if it is possible to change that message, i.e. from code injection?