There are a number of Google Translator plugins available but we usually use this one from Rob Myrick which seems to be kept up to date.
Step 1: Installing the plugin
You need to be a network administrator to undertake this step. From the Network Admin Menu go to Plugins > Add New and in the search box enter "Google Language Translator". From the presented plugins be sure to choose the one written by Rob Myrick and click "Install", then "Network Activate".
Step 2: Configuring the plugin
Having completed the first step the plugin is now available on every blog in your multisite and individual blog admins can configure the widget for themselves. On any blog that you wish to display the Google Translator widget go to Settings > Google Language Translator
As a minimum, leave the settings as they are, but simply click the languages that you wish to be available. We also recommend changing the default flag display for English to the Union Jack.
There are many settings that you can experiment with to display the tool as you wish on your blog.
Step 3: Displaying the tool on your blog
Now go to Appearance > Widgets and drag and drop the Google Language Translator into the required position in the sidebar of your blog.
Here's a short video summarising the above: