If you upgraded to the new FeedDemon 4.0 and English isn't your primary language, then you'll be pleased to hear that Chinese, Czech, French, German, Italian, Russian, Slovak and Ukrainian language files are now freely available.
The simplest way to switch to a different language is to select Tools > Options > Language > Download Additional Languages from within FeedDemon 4.0, which takes you this page:
From there you can select the language file you wish to use, and FeedDemon will take care of downloading and installing it.
PS: These language files were created by customers who simply wanted FeedDemon to be available in their native language. It's a tedious process creating these translations, so we owe a big "thank you" to those who spent their time working on them.