Normally there are two types of subtitling streams.
Forced and Unforced...
Forced streams are when a language other than the normal appears onscreen, and is not displayed throughout the rest of the movie.
Unforced streams are for hearing impared or for alternate languages.
The problem you are having could be because the disc was manufactured with another language in mind (ie german) and the forced german subtitles are not allowed by your DVD player by default. Alot of discount stores & ebay sellers now sell movies from other european countries, and it isnt until you watch a movie like this that you discover the type you have.
Some of the more expensive DVD players allow you to select forced subtitle streams aswell as unforced check your user manual or manufacturers web pages.