How is a study's default locale used?

The default locale for a study is used to store study metadata that can be localized.

After modifying the default locale, you do not need to restart the Central Designer application.
If you do not set the default locale, the regional setting is used. The regional setting is set in the Control Panel > Regional and Language Options > Regional Options tab > Standards and formats section. If you modify the regional setting, you must close and reopen the Central Designer application to use the new locale.
You select from the same list of locales that are available when you specify a regional setting.

Display properties that the default locale affects

The default locale affects the display properties of the following:

Some fields, such as the Default Question for a form, display the text for the default locale.
The title bar in the workspace displays the default locale. For example: Form : Demographics -- en-US
Float numbers are formatted according to the numeric standards for the default locale.

Typing translated data

You can enter translated data in the following ways:

You can enter translated data without specifying the locale, and the data is saved as data for the default locale. For example, you can enter the Default Question of an item in the Item Editor > Design tab > Default Question field.
You can enter translated data explicitly for a locale, and the data is saved as data for the specified locale. For example, you can enter the Default Question of an item in the Item Editor > Design tab > Languages section > Question field.

Making sure that the language in which you develop a study is the default locale for which study metadata is saved

To design a study for any locale, you must make sure that:

In the Central Designer Administrator application, the locale is selected.
In the Central Designer application, the locale is selected for the study.
The user profiles in the Central Designer Administrator application list the locale as a skill.