Would it be possible for an Earth-sized world to speak a single language?

Yes, it is absolutely possible.

There are 3 easy ways to achieve this I can think of right now:

1: A civilization could advance to a point were global communication is as easy as (or easier than) it is in current day earth and then through some referendum decide to change the official language everywhere to a single global language. Such a change would not happen overnight, and it is very likely that, at least for the first few generations but possibly forever, the 'official' language is just a shared second language. EDIT: It doesn't necessarily have to be an official referendum, it is plausible that given enough cultural/technological/political influence a certain language can in time be spoken by (almost) everyone.

2: Given that most lifeforms start at a certain point and then spread out. If your dominant species never expands further than they can reasonably communicate, they would most likely speak the same language. The most viable reasons for this would be an obstacle preventing expansion that was naturally removed after the species developed a language and afterwards enough contact was kept to maintain similar language (for example huge walls of ice) OR an obstacle overcome by technological advancements that are at a level where long distance communication seems simple enough to quickly implement.

3: The planet is inhabited by a subgroup of a species from another planet speaking the same language that already had the technology for planet-wide communication upon arrival.