Going by the idea of the Kardashev scale I can imagine that if humanity reaches the point of being a globally unified Type I civilization then by necessity there would be one main language everyone would be using. Extending the idea of the Kardashev scale to humanity becoming a Type II civilization in which there are different colonies in the solar system maybe the same language would still be used. Going even further to Type III I would start to doubt whether humans spread throughout the Milky Way galaxy could maintain a unified language. If faster than light communication could ever be possible then maybe a unified language could still be maintained.
It's really hard to say without speculation. Right now there are countries like North Korea that use their own unique calendar. The international standard is the Gregorian calendar while some cultures still use a lunar calandar. It's hard for everyone to agree on a language when we can't even agree with an answer to "What is today's date?"