Players playing Minecraft in german will get german messages and english players english messages.
If you want to help translate MyPet you can do so on the MyPet Translation Site. MyPet is already translated into several languages so please check the site for existing translations before you do your own.
You can create your own translation by modifying an existing translation or creating a completely new one. Either way you have to put the file into the locale folder and name it in this schema: MyPet_XX.properties . XX has to be replaced by the locale code of the language you want to change/create.
Overriding the Language
If you don't want the default behavior where the plugin will send messages in the language of the player or you did your own messages then you can force the plugin to use only that one language. You can do so by setting OverwriteLanguages in the config.yml to the locale code you want to use.