"You should study this languge..."

"...first, before starting to study that one."

People who say that, DIAF.

>>5
http://en.wikipedia.org/wiki/Propaedeutic_value_of_Esperanto

See particularly the "Provincial Grammar School in Sheffield (GB)" section, which has quite clear results.

I don't know about adults, but It wouldn't surprise me if a few months of Esperanto achieved a similar result.