Based on Steam’s hardware survey, over 37% of users have the language set to Simplified Chinese.
Every month the optional survey collects data about the hardware of participating users, things such as the CPU and GPU used, free space on the PC, but also the user settings on Steam. Until December 2019, the most common language was English, being used by between 30% and 40% of users.
There were some issues with the survey in previous years, specifically duplicate results coming from Chinese internet cafés. This is a bit of a problem, since this is the largest survey of its kind. One of the most common inconsistencies was the number of users with AMD or Intel CPUs compared to actual market share. Valve did say they managed to fix the issues though.
Back to the numbers themselves, Chinese has overtaken English at 37.8% of overall users, which is 14.4% higher than the previous month and dropping English to 30.4%. It doesn’t come as much of a surprise considering that the Chinese government heavily regulates and censors games, even going as far as not giving out publishing licenses for new games, but Steam somehow circumvents these restrictions. So it comes as no surprise that Chinese gamers are abundant on Steam.