Is it normal that TFormatSettings.Create('en-US') returns with ShortDateFormat = 'm/d/yy' on Android but returns with ShortDateFormat = 'M/d/yyyy' on Windows ? Is it a bug? I was thinking that the settings must be consistent against platforms.
TFormatSetting uses platform-specific settings whenever possible, even when using the same locale across platforms.
On all platforms, TFormatSettings.Create(LocaleName) converts the LocalName to a platform-specific TLocaleID and then calls TFormatSettings.Create(LocaleID).
On Windows, the LocaleName is converted to a TLocaleID (alas for LCID) via LocaleNameToLCID() on Vista and later, and EnumSystemLocales() on earlier versions. The LCID for 'en-US' is 1033. 'm/d/yy' is the RTL's default if the Win32 API GetLocaleInfo() function does not return a LOCALE_SSHORTDATE string for the specified LocaleID. On your system, Windows is returning 'M/d/yyyy' for LCID 1033, so the RTL's default is not used.
On Android, the underlying localization library used by the RTL is the cross-platform ICU library, so the LocalName is converted to a TLocaleID by simply encoding it as UTF-8 and then using that as-is. 'm/d/y' is the RTL's default if ICU does not return a UDAT_SHORT string for the specified LocaleID. On your system, ICU is returning 'm/d/yy' for 'en-US', so the RTL's default is not used.