Gantt charts are localized to your language.
It uses the user’s Atlassian account preferences which are settable at https://id.atlassian.com/manage-profile/account-preferences. So, each user can use their own language.
Some languages support localizing texts and date format while the other languages only support date format localization. If your language is not supported, English(US) is used.
Japanese (Both texts and date format localization are supported)
Dutch (Only date format localization is supported)
If your language has or is not in the table below, English(US) is used.
Text translation supported language will increase step by step.
And, if Confluence Cloud increases supported language, the app will support it as much as possible.