Editing the Language Selection 
The Phrase Management: Language Selection dialog box appears.
You can set the C (comparison language) indicator for the language that is to be used as comparison language for translation in the long text editor. The phrase is then displayed in the comparison language in the long text editor. If you do not set the indicator, the source language of the phrase is used as the comparison language.
Result
The settings you have made are transferred to the Edit phrase: Items screen. The settings remain until you have exited the phrase editing transaction.