If a form is submitted indirectly by another application, then the form must contain a hidden input field to specify the language parameter. The field's name is LanguageParam, and its value specifies the ISO 2-letter code for language and region.
For example, the LanguageParam field for English-United States would be the following:
<INPUT type="hidden" name="LanguageParam" value="en-us" />
The LanguageParam parameter is required in certain situations. Forms submitted through Chrome (or an automated process, such as cURL) require the parameter to pass in the locale, but forms submitted directly to LoginFormProc from Internet Explorer, Safari, or Firefox do not require the parameter. If a form submitted through Internet Explorer, Safari, or Firefox contains this parameter, then the parameter takes precedence over the browser's locale.
In certain instances the HTML form is not submitted through LoginFormProc directly. For example, a third-party application may be used to process the data and then submit the post data to LoginFormProc. In this situation, the application is required to pass the LanguageParam in as part of the post data.