"A foreign language is the skill he could not master since his childhood."
I don't think this sentence would be grammatical. Guess it should have been "he hasn't been able to master since his childhood". Or if the person the speaker is talking about is dead than ought be as follows "A foreign language was the skill he had not been able to master since his childhood."
A foreign language is not a skill. The ability to learn one is.
'Learning a foreign language was a skill he had not been able to master - even from his childhood.'
I still don't like it much.
I bet somebody else can put it better.
Rover
If we are talking about the past, can we possible say
"Learning a foreign language was a skill he was unable to master from his childhood"?
In this case, it probably means that he never mastered the skill.
"Learning a foreign language was a skill he had been unable to master from his childhood, until one day he met Mrs Johnson".
Is the 1st sentence correct?
It might have to be discussed in a separate thread, but I don't see any problem with that sentence. Saying that "a meeting discussed something" implies that "those who took part in the meeting did the actual discussing".