、。
Mastering French in two or three years is extremely difficult.
Literal
2, 3 years [in-で] French [target-に] become-proficient [nominalizer-のは] extremely difficult.
Uses the cleft-like structure [clause]+のは+[predicate], with の as the nominalizer. 熟達する ('to master, become proficient') is a formal verb — 上達する is the more common equivalent. The target of mastery is marked with に, not を — because 熟達する is an intransitive 'reach proficiency at.' きわめて ('extremely') is a formal intensifier.