Popular.
Ok, there are a lot of synthetic or constructed languages. Many people here have already pointed out Esperanto.
Too bad there are significantly more people that speak Klingon than Esperanto. Esperanto is a failure.
What would it take other than being popular? Making it common, useful, or even important. Require it to be taught for school children and free classes available for adults. Then make things people want or need only available in that language. Some options include government services, others might be 2/3s of a media stations programming, get creative. Preferably, do it in many countries, especially 1st world nations, at the same time.
Would people rebel against that? Oh hell yes! Just look at metrics in the US. Most people still have no freaking clue how many centimeters are in a meter (here's a hint, the metric system is based on 10s, with decimeter being between centimeter and meter) and they were teaching that thing to school kids since the 60s that I know of! Most of the world, and all of the other 1st world nations, have already officially switched to metrics, even the UK. In fact most sources are only listing 3 countries that haven't officially switched yet.
To put some perspective on that, the metric system is reported to have been created in 1799... That's around TWO HUNDRED AND SIXTEEN YEARS! And that's only a measuring system, not something even vaguely as significant and large as a language.
So yes, the only way to get a new language adopted is either find some way to make it so totally popular everyone wants to know it so bad they'd skip their own birthdays and sex to take even one class. Or to force everyone to learn and use it while giving it some modicum of popularity and usefulness.
So anyhow, good bloody luck with that.