Hi everybody,
If you start creating your component in your own non english language, you will soon discover that many keywords in the generated language files will be in your own native language. Cook simply takes your text entries and creates keywords for the translation files. So you will get entries like:
PROJECT_FIELD_MONATS_KALENDER instead of PROJECT_FIELD_MONTHLY_CALENDAR
So from my experience now, it is advisable to create your component entirely in English, then all the keywords in the translation files will be properly set in English. So you can create your own language files at the end of the process. This is particularly important if you are going to distribute your component and want the Joomla community to create translation files for you.