Developing site-specific Modules in non-English language

A general/strategic questions regarding translations approach:

For my site I will have to develop certain site specific modules which can not be contributed. The site will first be in German only, if successfull other languages may follow later. I watn to avoid having the text occuring in the module in English at this stage and would like to have my modules for German only, first, using of course the t() function.

As core modules are in English and my modules would be in German I am afraid I run into trouble if I want to tranlate it to English later because the t() function will not know if the input-string is English (core modules) or German (my modules). What approach would you recommend?

Thanks.