There's no configuration in Tempo Timesheets to change the language. The language can be changed in the Jira user profile.
There you can select the language you would like your instance to display for you. Please take also a look at https://confluence.atlassian.com/jiracoreserver/managing-your-user-profile-939937934.html"managing your user profile" ( for Server/ Data Center)
or https://confluence.atlassian.com/get-started-with-jira-core/manage-profile-and-preferences-867014216.html"manage profile and user preferences" ( for Cloud) Please note, that as of October 2020, Tempo Timesheets and Planner in cloud are also supported with French translations in the interface. More languages are currently in the Roadmap to be added in the near future. | 
