There is a section of the TeXWorks manual in which there is the reference to a guide on how to use dictionaries.
The link it refers to is this, so you should place your personal dictionaries in the folder /usr/share/myspell/dicts.
One easy way to add them is to install them.
I suppose that if you use LibreOffice then they are installed by default.
If not, you can simply install the packages named myspell-XX, where XX stands for your language.
You can easily find them with apt-get and auto-completion or via Synaptic.
Use Synaptic, apt-get or any other package manager to install the package hunspell-de-de.
This is the spell check package for German that LibreOffice uses.
From the LO Writer menu, select Tools > Options
In the dialog, choose Language Settings > Languages
You can change your interface and locale languages to German if they are not already set, but the important one is: Default languages for documents (Western). Make sure this says German and hit OK.
Restart LO Writer.
That's it. New documents should default to German and use the German spell check.
Note that old documents will still be set to English. To change an existing document to German, from the menu: Tools > Language > For All Text > Reset to Default Language
Best Answer
It seems to me that skype for linux doesn't provide spell-checking at all. see this link: http://community.skype.com/t5/Linux/Spell-check-in-newest-Skype-for-Linux/td-p/1050184