From the fine manual:

See the CultureInfo.CurrentCulture property to learn how a thread's default culture is determined, and how users set culture information for their computers.

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How a Thread's Culture Is Determined

When a thread is started, its culture is initially determined as follows:

• By retrieving the culture that is specified by the DefaultThreadCurrentCulture property in the application domain in which the thread is executing, if the property value is not null.

• By calling the Windows GetUserDefaultLocaleName function.

Note that if you set a specific culture that is different from the system-installed culture or the user's preferred culture, and your application starts multiple threads, the current culture of those threads will be the culture that is returned by the GetUserDefaultLocaleName function, unless you assign a culture to the DefaultThreadCurrentCulture property in the application domain in which the thread is executing.

For more information about how the culture of a thread is determined, see the "Culture and Threads" section in the CultureInfo topic.

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Culture and Threads

When a new application thread is started, its current culture and current UI culture are defined by the current system culture, and not by the current thread culture. The following example illustrates the difference. It sets the current culture and current UI culture of an application thread to the French (France) culture (fr-FR). If the current culture is already fr-FR, the example sets it to the English (United States) culture (en-US). It displays three random numbers as currency values and then creates a new thread, which, in turn, displays three more random numbers as currency values. But as the output from the example shows, the currency values displayed by the new thread do not reflect the formatting conventions of the French (France) culture, unlike the output from the main application thread.

Note that it says when a new application thread is started. So, given that you say:

It's not from the Windows culture info because I have changed that from en-US to en-GB without noticing any change in the application when running.

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