Let me explain something, and this is in no way condescending.
The way programs work is defined entirely by the programmer, and the programmer may only have intended the program for use by one particular language group, so the program may end up with hard-coded strings and text in a specific language, which would be hard to translate without rewriting the core of the program.
Also, many programmers don't know several different languages. It's hard enough for them to learn one programming language without having to learn more than one actual language. So while they may want to learn another language or provide translation services for their application, they can't because they may not know them or be able to learn them to provide an effective translation.
And so the dilemma - programmers simply don't have the knowledge to add translations to their program or the ability to provide said translations. If you have an app you like, but would like to see in a different language, why not contact the author and provide your services in terms of translations? I know I did.