It's all in the title.
By language I mean human language that are not included in the default version (say dutch, german, spanish, ect...). I'm using gedit 3.4.x.
I found an article that may be useful: https://help.gnome.org/users/gedit/stable/gedit-spellcheck.html.en
This states that you can Define the language to use for spell checking
By default, gedit will use your current language to spell check your document. If you need to use another language, choose Tools ▸ Set Language…, then choose the language you want to use.
EDIT: Sorry, I overlooked that a bit. I think you need to install dictionaries for them to show up in Tools > Set language menu I believe the languages will show up after you install the myspell-nl dictionary using System > Administration > Synaptic Package Manager
