The 24-year-old will have his Forest medical on Tuesday with the deal set to take Forest’s summer spending to £73.5m.
The Belgian international made 29 appearances across all competitions for the German side last season, scoring one goal.
Mangala also made his Belgium international debut in March against Ireland.
Forest are also still working on a deal to sign Real Betis left-back Alex Moreno, who made 45 appearances in all competitions last season for the La Liga side.