I have just installed Ubuntu and by mistake I made the installation in English. I would like to change it to Spanish. Can anybody explain me how to do it, please??
Click on system settings --> Language Support
You may be asked if you want to update or add components to your currently installed language(s) when you first open the dialog. Click Install to go ahead and install the additional components to complete the localization and avoid to be prompted next time you start Language Support.
Scroll down to Spanish, and Click Install / Remove Languages to add it.
Once you’ve made your selection, click Apply Changes to install your new language. Make sure you’re connected to the Internet, as Ubuntu will have to download the additional components you’ve selected.
Type in your password, and then click Authenticate.
Drag Spanish to the top of the list, and click on Apply System-Wide.
Once you have your new language settings all set, go ahead and log out and log back in. Once logged-in, Ubuntu may ask you if you want to update your user folders’ names to your new language, click Update.
Then you can add a Spanish keyboard layout, just follow the following images to do so.
Once done, go back into Language Support to change the Regional Format. See image below.
Go to Settings->Languages and move Spanish to the top of the list. Then install the suggested packages.
If you're on Ubuntu 14, go to System Settings. Under Personal, you'll find Language Support, there will be a button on the Language Support window that says, Install/Remove Languages. As the person above said, just drag the new language to the top from there.
[The only reason that I'm adding an answer, when this question already has one, is that the path to get to the language settings is different in the new Ubuntu.]
This question is best answered by referring to the official docs.