Isn't it the language of the locale of system the application is run in?
I was hoping to get an actual answer from someone who has had to do it. If you are correct, and I hope you are not, in order to _see_ their chosen language rather than the language on their computer, one would have to change the language with QLocale which would not change the language for the external processes being communicated with via pipe or some other means.

AFAIK, QString is a unicode format. Each text returned from any Qt item is of a QString type. Unless you do not do any conversion, you can type in any language and everything should be correct.
you seem to mix two things here I think.
I'm not mixing two things. I'm talking about multiple things. I'm talking about an application which must communicate with multiple completely independent services, some local, some on the Web, which will store to databases expecting English.

Having an application which displays a users chosen language for prompts and such via translation, but shows English because that is the locale of their machine would be incredibly tacky. Likewise, if you have fields like name and address which get entered in a language other than English yet has to be sent to processes and services which expect English, it is a significant problem. I can see no easy method of translating such things to English because there is no hard coded string to use with a qm file.

Yes, many databases will store unicode these days, but that also presents real problems.

Take, for example, a made up username like al-Ahammed
If one user enters it in English it looks as above. If another user enters it in Arabic it is a completely different set of unicode values. Likewise if another enters it using Kanji, etc. This presents significant query issues for technical support when they are trying to find and fix the account for one of the other languages.

Right now this is just a theoretical problem as I'm kicking around a system design which may or may not see the light of day, but this is a significant issue for anyone trying to develop an application for world wide use that doesn't want to force English down the throat of every user. I consider it horribly tacky for a system to allow a user to choose their application language, display all of the prompts, buttons, and help text in that language, then force them to enter English into any field which may be sent to a Web site or a database. There are a large number of users in the U.S. who leave their computers set to English but set specific applications to display their native tounge.

Once again, I would like to get an answer from someone that has actually written a multi-lingual application using Qt. Especially if that application MUST transmit English to some external system.