1
general::
Understand one another very well, agree with each other, as in Negotiations went on for days, but finally both sides realized they weren't speaking the same language. This term, alluding to literal understanding of spoken words, dates from the late 1800s.
American Heritage Idioms
2
general::
Phrase(s): speak the same language
1. Lit. [for two or more people] to communicate in a shared language. • These two people don’t speak the same language and need an interpreter. 2. Fig. [for people] to have similar ideas, tastes, etc. • Jane and Jack get along very well. They really speak the same language about almost everything. • Bob and his father didn’t speak the same language when it comes to politics.
McGrawhill's American Idioms And Phrasal Verbs