In 6.0 Zabbix has switched from en_GB to en_US as a default locale. en_US is fine as a default language but it also includes date and time formats that are not very common outside of the US. Please add en_GB locale back (with 24-hour time) as a separate choice so:

• British users can add their translations
• English-speaking non-US users have the option to use alternative date and time