All the texts you can see in the application can be translated online via Transifex web application. You just need to sign up to this service, visit the Planisphere project and ask for creating a new localization group for your language (or join any current one).
There is a single localizable resource for the Planisphere project called messages. When it is selected and appropriate language is chosen, the following interface is displayed.
On the left there is the list of all the texts available. On the right there are details for the every particular item. It is highly advisable to display More details to enable displaying the Key (highlighted in the image). The Key helps in understanding the context.
In the example above we need to translate N. Thanks to the Key it is clear we are translating the north cardinal point abbreviation. When S (czech localization) is entered into the appropriate box and Save button is pressed, we can continue to the next item.
After completing and final reviewing the translation (by administrator) it is automatically stored in the GitHub repository.
When you are willing to start a new localization from the scratch and the description above seems to be too complicated, you can alternatively download the English version as a text file (messages_en.properties) and edit individual entries directly in your favorite text editor. Just send the result to my e-mail address and I'll manage all updates by myself.
The online application is updated usually in next few days.
In case of any questions do not hesitate to ask.
Thanks in an advance!