The extension is called Google Input Tools. You'll be able to type in a different language with the click of the mouse and switch back just as easily.
After installing the extension you need to set it up. Click on the extension icon and select Extension Options in the menu. A new tab will open. Here you can select the languages or input methods to meet your needs. You can always return to this selection page to add or remove language options.
To use Google Input Tools, simply click on the extension icon, select the desired language and then click on any text box on the web page (or in a Google Doc) and type. To turn it off, simply click on the extension icon again to switch back.
I know you're probably thinking, "I already use Google's translating feature." That is obviously an outstanding feature. However, in certain languages, a literal translation from English oftentimes does not make sense. This extension allows users to convert sounds of languages into correct words, not take the meaning. For example, to type the characters for the word "hello" in Chinese (你好) you have to use the sounds "ni hao," and then hit the space bar to pull in the correct characters.
If you can write in a different language, there are also handwriting options available through the extension.
For a quick review about the extension, check out this video.