> Until now I worked with Emacs to edit ConTeXt documents. Because I have to make a manual for the client, I now work under Windows with TeXworks. In Emacs I could easily reformat my (source) lines, to be not wider as 72 characters. Is something like that possible with TeXworks.

You can change it in the menu bar: Format -> Hard Wrap…

> Also: there is a spell-checking option, bet there are no languages to choose from. How can I install languages?

Take a look at the TeXworks manual: http://www.tug.org/texworks/#Documentation