The Client's interface will by default take the language that has been set in the operating system. If a user needs to change the client interface language, this can be done as a personal User Preference setting.
In the Designer client, go to User Preferences in the client's upper right corner:
In the Anywhere client, language settings are done as part of the login:
If necessary, you may need to log out from the Anywhere client first:
Note: Changing the client language will also change the language of the meta data (names of dimensions and measures) if these have been translated. See this article for translation of meta data: Language - translations of metadata, folders and documents