TeXStudio uses the OpenOffice spelling dictionaries (as well as thesaurus). You can download any number of them from the OpenOffice site. The OpenOffice files have the extension .oxt but are actually .zip files so just change the extension and extract the dictionary. You place the dictionary files in the Dictionary folder in your TeXstudio installation. The disctionary then appears in the list when you open the "Configure TeXstudio" under "Optons" in TeXstudio.

EDIT: For weird reasons, the dictionaries link from the given link doesn't work. Instead use the following link: List of dictionaries. So here is the steps:

  1. download your dictionary (.oxt)
  2. change the extension to .zip
  3. unzip the files in a folder
  4. In TeXstudio preference, under Spell Checking Dictionary, change the directory to the folder you put your dictionary in
  5. reset TeXstudio.
  6. You are all set!