@IkaPlusDepending on your organization's Microsoft 365 configuration, you might find Settings in a different place. > Microsoft 365 settings, and select the Settings tab. Select Language and time zone and choose the language you want. Then select Save
The Planner tool is always based on the language settings of the user. any language is used for the bucket descriptions, task descriptions, etc. will always be based on what the creator has used but there is no language setting for plans.