according to this study: http://www.pwc.com/gx/en/world-2050/assets/pwc-world-in-2050...
Mexico will be the 7th economy in the world by 2050, Spain and Argentina 15th and 20th respectively.
In the US 1/4th of the population will be Hispanic in 2050, although this doesn't mean that 1/4th of the population will be Spanish native speakers, at least promises a high level of Spanish proficiency, source: http://www.pewresearch.org/fact-tank/2014/12/16/with-fewer-n...
If I could bet I'd put my money on Spanish.