How to install a LibreOffice Language Tools (Spelling Check, Thesaurus...)

This is apparently managed at the system level. Go to the Language Support menu in the System Settings menu and add the languages you want to support. After that restart LibreOffice and you will see that the spellchecker for example will give you the option of using the newly installed languages.

The command: sudo apt-get install aspell-<language pack> will do the trick

For example, the spanish dictionary is:

sudo apt-get install aspell-es The command: sudo apt-get install aspell-<language pack> will do the trick

For example, the spanish dictionary is:

sudo apt-get install aspell-es

In Kubuntu the package is called myspell-es

If you can't install system packages, you can install a spell checking dictionary extension manually. You'll need to restart LibreOffice.

US English Dictionary available here: extensions.libreoffice.org.

Instructions for installing extensions:

1. Download an extension and save it anywhere on your computer.
2. In LibreOffice, select Tools → Extension Manager from the menu bar.
   
3. In the Extension Manager dialog (Figure 2), click Add.
4. A file browser window opens. In Add Extension(s) dialog, you can find the extension's files in your system folders. The extension's files have OXT file extension.
5. Find and select the extension you want to install and click Open.
6. If this extension it is already installed, you'll be prompted to press OK to confirm whether to overwrite the current version by the new one, or press Cancel to stop the installation.
7. After, you are asked whether to install the extension only for your user or for all users.
   • If you choose Only for me option, the extension is installed only for your user (the extension will be stored in your user profile and other users will not have access to it).
   • If you choose For all users, you must have system administrator rights. In this case, the extension is installed in LibreOffice system folder and will be available for all users.
   • In general, choose Only to me, that doesn't require administration rights on the operating system.
     
8. After, you may be asked to accept a license agreement.
9. Use Scroll Down button to read the license agreement. At the end of license text, the Accept button will be enabled.
10. Click Accept to proceed the installation.
11. When the installation is complete, the extension is listed in the Extension Manager dialog.

Instructions stolen from libreoffice.org