Josh and Cécile are correct -- each user can select his/her own language/locale/time zone preference via "My Account". Screenshots and more guidance are available here: https://community.marketo.com/MarketoArticle?id=kA050000000L8k8
(I think the confusion may be that "My Account" is shown under "Admin" in the tree, but non-admins can access "My Account" by clicking on their name, as Josh described above.)
Also, you may want to take a look at this article What to Expect if your Language Preference Differs from the Default
. "Default" references the subscription language (or workspace language, if your company uses workspaces).