Per default the vSphere Web Client will use the language definied in the operating system or in your webbrowser.
If you want to change it, just add the following localization parameter to the URL you use to start the Webclient:
to start the vSphere Web Client in English:
https://IP_of_vCenter:9443/vsphere-client/?locale=en_US
to start the vSphere Web Client in German:
https://IP_of_vCenter:9443/vsphere-client/?locale=de_DE
to start the vSphere Web Client in French:
https://IP_of_vCenter:9443/vsphere-client/?locale=fr_FR