Is just like Gaven said, and remember that you usually do not assign any key, just put something like <#_ the sentence#> and it will create automaticly the message key in the catalog (never touch by hand the files in the catalog directory in your package root). Try to translate the whole sentence instead of word by word, in that way we can say that a package is really international because is adapted to any kind for language construction.
If you are dealing with date this might be useful: https://openacs.org/forums/message-view?message_id=70680
