<p>We are developing a web application which is available in 3 languages. </p>

<p>There are these key-value pairs to translate everything. At this moment we use Excel (key, german, french, english) for this. But this does not work well ... if there is more than 1 person editing this file, you have no chance to automatically merge the different files.</p>

<p>Is there a good (and free) tool which can handle this job?</p>

<p>--- additional information ---</p>

<p>(This is a STRUTS application) But the question is how to manage these kinds of information in general (or at least in an conveinient way, which also supports multiple users editing this single file ("<em>mergeable</em>" filetypes))</p>

If you can store this information in plain text then you will be able to use a version control system like subversion to help you with merging changes.  Subversion is free.
The free guide (the "Red Book") to subversion gives a fairly good explanation of how this kind of merging works.
http://svnbook.red-bean.com/en/1.5/svn.basic.vsn-models.html#svn.basic.vsn-models.copy-merge
EDIT: Another thought - if you really want to stay using a spreadsheet - Google Docs supports simultaneous editing of a spreadsheet.  You could import your existing spreadsheet and get your multi-user merging wishes for free with very little change to how you work.