* Sensitive issue amid merger talks with Commerzbank (Updates with further details, background): FRANKFURT (Reuters) - A plan by Qatar’s sovereign wealth fund to make a significant investment in Deutsche Bank has stalled, according to two people with knowledge of the matter. The Qatar Investment Authority (QIA) approached financial regulators months ago to seek approval to buy a big stake in Germany’s largest lender, the people said. Qatar’s royal family already holds a combined 6.1 percent stake in Deutsche Bank. But Qatari officials haven’t yet provided the necessary documentation, which could derail the QIA’s investment plans, they said.