It is indeed a problem for German millitary to be involved into such a operation. The thing is that it would probably part of the mandate to protect Lebanese civillians (as well as those of Israel) from armed hostillities. This is the reason why the UN peace troop will be no direct "ally" of Israel but a neutral power between Israel and Libanon. Therefore UN troops could be engaged into a conflict with Israeli troops in case of their retaliation for possible terror acts of the Hezzies.
We contemporary Germans stick to the conditions and the words of the treaties we sign. Since we are no "Dutchbatters" (if you remember the Dutch troops that betrayed the civillians they should protect from their Serbian enemies), it is better to stay out a conflict were we a no neutral force. Germany understands itself as a ally of Israel. Since it is completely UNTHINKABLE that German forces have armed hostilities with the troops of Israel my prediction is, that the majority of the German parliament, the German Bundestag, will be against any participation with land based combat troops.
It will be thinkable to help both Israel and Lebanon with armed Federal Policemen (German police was always very effective in such cases) to control the border of Syria and with the deployment of German frigates to check the maritime traffic of Lebanon to chop the weapon deliveries to Hezbollah from Iran and Syria. A direct deployment of German forces between Israels boarder and the Litani river is rather unlikely.
Besides - I heared in German media that it will be difficult to get those 15.000 men togehter, since most nations do simply lip service and want to keep out of this conflict, although the nessecary resources would be availabe in Europe. We will see more tonight when the conference of those nations, who will deploy the troops, will be over. Maybe we undergo a positive surprise.
:-)