There are site settings for forums which are adjusted depending on the value selected in the default locale site setting. They don’t depend on the readers’ or writers’ preferences, but help the admin to configure a forum for a specific language. You can read more about this here: Locale-specific defaults for site settings (`locale_default`)
Admins can still configure all those settings as they want. So an admin of a site where the default locale is English can still allow shorter titles and posts or the use of more characters in usernames. It is only the default of the setting that might make the most sense that changes. So also on a forum where the default locale is English, the admin can configure the enable_inline_emoji_translation site setting so no space is needed.