I live in Poland and speak Polish but tend to have my devices set to English because of how low quality UI translations tend to be. The endless confusion this causes to software is amusing. Worst/best of all is Google Maps on Android, in which random parts of the interface, including voice directions, can independently be in Polish or English, and which is which changes on its own on a daily basis.
Windows is notorious for this---its code page (i.e. character encoding and thus effectively language and script) is directly tied to the locale. Most importantly this meant the same code page 950 (Big5) was used for both Taiwan and Hong Kong. Hong Kong had its own extension to Big5 called HKSCS, so they could be distinguished only by the OS edition. (Internally Big5+HKSCS has a separate code page 951, but AFAIK this wasn't exposed publicly.)