The Original Poster's Japanese friend has a very good reason to ask this question, and if we don't understand what this is, then we cannot fully answer the question.
We usually need a reflexive pronoun when two noun phrases in a clause refer to the same being or entity:
These don't need to be the subject and object of the clause:
In the first example above the reflexive himself is the complement of a preposition. In the second, the original referring expression, Bob, is an object, not a subject.
Now, we also need to use a reflexive if a pronouns co-refers with the UNEXPRESSED SUBJECT of a clause. For example, consider the following:
- Take a look at yourself in the mirror. You look great!
Here the unexpressed subject of the imperative clause is you. Because the pronoun after the word at refers to the same person as the person who's going to be doing the looking, you, it must be reflexive. A reflexive pronoun is also required in the following example:
- By teaching yourself English, you can massively improve your employment prospects.
Here the understood subject of the verb teaching is you. We can rephrase the sentence, rather awkwardly, like this:
- By your teaching yourself English, you can massively improve your employment prospects.
So the person being taught is the same as the person doing the teaching. Because yourself is in the same clause as the unexpressed subject, it needs to be reflexive.
Now let's have a look at the Original Poster's sentence:
- Learning foreign languages changed me.
Here the unexpressed subject of the verb learning is me. The object of the verb change is also me. So the question is why isn't the object of the verb changed required to be reflexive here, given that it occurs in the same clause as the verb learning—which has a co-referential, though unexpressed, subject?
One extremely bad answer would be to say that the subject of the verb changed is not the same as the object of the verb changed. This is a bad answer, because as we have seen from the Bob had a picture of himself example, if a pronoun refers to the same entity as a noun phrase in the same clause, it is usually required to be reflexive regardless of whether either of them is the subject or the object. In the photo example, himself is not what is being given.
The real answer to the Original Poster's question is that we only need a reflexive pronoun if it occurs in the same minimal clause as a co-referential noun phrase. In other words they must both occur in the same smallest clause. In the following example he occurs in a smaller clause inside the larger sentence and the noun Bob is not inside this same small clause. For this reason no reflexive is required:
- Bob believes [he is a great singer].
Similarly, in the example given by the Original Poster, the unexpressed subject of the verb learning is in a smaller clause inside the larger sentence. The pronoun me does not occur inside this clause and isn't required to be reflexive:
Me learning foreign languages] changed me.