Viewer and Aligner > languages
Each Viewer file (.VWR) has its own language. Each Aligner file (.ALI) has one language for each of the component sections. (They could all be the same, if for example you were analysing various different editions of a Shakespeare play they'd all be English.) The set of languages available is that defined using the Languages Chooser.
If you find you have read in a plain text without defining the language correctly, you can change the language to one of your previously defined languages by pressing the 
button visible at the top of Viewer & Aligner.