According to Rice's Theorem
Any non-trivial property of the LANGUAGE recognizable by a Turing machine is undecidable
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For a property to be non-trivial, there should exist at least two Turing machines, the property holding for the language of one
Tyes and not holding for the language of other Tno.
So we can make Turing machine for L and L' in this question,so it is undecidable.