Pronoun: one vs it

According to the following explanation of using ‘one’ in Oxford Practice Grammar:

We use ‘one’ to talk about an object in general (6) and ‘it’ for a specific example of an object (7):

(6) Do you have a French dictionary? I am looking for one (= not a specific French dictionary) (7) Do you have the French dictionary? I am looking for it (= a specific French dictionary)

Now, I have a question : The following context is extracted from another book:

Learning a language as a child seems to involve no effort, but learning it as an adult requires a lot of time and effort.

In the above sentence a language is used in general, but why does the writer use it instead of one?

In the sentence

Learning a language as a child seems to involve no effort, but learning it as an adult requires a lot of time and effort.

a language has been mentioned in the gerund clause, so it may serve as an antecedent for it, which occurs later on in the sentence. There is therefore a slight implication that the two languages are the same, as in

Learning English as a child seems to involve no effort, but learning English as an adult requires a lot of time and effort.

The sentence

Learning a language as a child seems to involve no effort, but learning one as an adult requires a lot of time and effort.

uses the indefinite pronoun one, which does not refer to a specific syntactic entity in general (and in particular not to any entity already introduced in the sentence). Now the sense is that children seem to learn any language with no effort, but it's hard to learn any language as an adult. The languages need not be the same, as in

Learning a language as a child seems to involve no effort (even a difficult language like Basque), but learning one (even a comparatively simple language like Spanish) as an adult requires a lot of time and effort.