This Wikipedia entry has a table listing the world’s languages in descending order by number of speakers. Besides the quantitative information, the table also displays the countries in which the languages are spoken and provides some helpful notes.

For example, here’s the note for Arabic: “The Arabic language contains many different dialects. Most are mutually intelligible. See Varieties of Arabic.”

And here’s the note for Spanish: “It has some degree of mutual intelligiblility with Portuguese, see: Comparison of Spanish and Portuguese.”

I can’t vouch for the entry’s accuracy, but it is at least a good starting point for language research.