Complex sentence: Fort Maurepas was the capital of Louisiana (New France) in 1719, when the capital was being moved from Mobile (in present-day Alabama), while government buildings at New Orleans were still under construction.
Split into simpler sentences: Fort Maurepas was designated temporarily as the capital of Louisiana (New France) in 1719. The capital being moved from Mobile (in present-day Alabama) to New Orleans, but government buildings in the latter city were still under construction.