First, as you may know, automatic translators nowadays do not so much translate but look up people's (presumably correct) translations and try to infer patterns; that is, if a lot of available materials have a final subject pronoun in Spanish rendered into English as a reflexive pronoun, the automatic translator will use that, regardless of the logic.

Second, I tried looking for academic sources but found none specifically relevant to this, so I'll just explain based on what I've read here and there, plus my intuitions as a native speaker.

In any sentence there are two important positions: the beginning and the end. The beginning of the sentence is normally where the speaker introduces the theme or topic, the thing that the rest of the sentence will be about. After the topic comes the comment, a.k.a. rheme or focus, that is, the things that are said about the topic. Often the end of the sentence is the most focal position, where the really new information or unexpected facts are mentioned. All of this is very general; there are other things to be factored in. The topic is "old news", so it might be absent if the speakers has already mentioned it clearly before. The topic might be emphatic or contrastive. The focus can be very "soft", the sentence being rather general with no particular emphasis. That certain Spanish verbs call for verb-subject (VS) order rather than SV order has to do with focus (“Se cayó el jarrón” first says to you something has fallen, and then gives you the really important news: it was the vase!).

In the case of the final subject pronoun (it could be a full noun phrase too), the displacement puts the agent of the action into focus. “Lo prefiero comprar yo” has the old information first (the conversation is about buying pants) and then the news: it's not the other person who will buy the pants, but the speaker.

English cannot dislocate a pronoun in this way, so in the face of such a sentence, it has to resort to other means. With a simpler syntax it could be a cleft sentence, but here I suspect “It's me who prefers to buy them (the pants)” sounds terrible, and “The one who prefers to buy them is me” is ridiculous. The alternative is a reflexive pronoun added for emphasis: “I prefer to buy them myself”.

In Spanish it would be possible to use the reflexive as well, but the pronoun would be dislocated anyway: “Lo prefiero comprar yo mismo”. If the speaker left it in the preverbal position it would sound exactly like English “I myself prefer to buy them”: the meaning would be different.

First, as you may know, automatic translators nowadays do not so much translate but look up people's (presumably correct) translations and try to infer patterns; that is, if a lot of available materials have a final subject pronoun in Spanish rendered into English as a reflexive pronoun, the automatic translator will use that, regardless of the logic.

Second, I tried looking for academic sources but found none specifically relevant to this, so I'll just explain based on what I've read here and there, plus my intuitions as a native speaker.

In any sentence there are two important positions: the beginning and the end. The beginning of the sentence is normally where the speaker introduces the theme or topic, the thing that the rest of the sentence will be about. After the topic comes the comment, a.k.a. rheme or focus, that is, the things that are said about the topic. Often the end of the sentence is the most focal position, where the really new information or unexpected facts are mentioned. All of this is very general; there are other things to be factored in. The topic is "old news", so it might be absent if the speakers has already mentioned it clearly before. The topic might be emphatic or contrastive. The focus can be very "soft", the sentence being rather general with no particular emphasis. That certain Spanish verbs call for verb-subject (VS) order rather than SV order has to do with focus (“Se cayó el jarrón” first says to you something has fallen, and then gives you the really important news: it was the vase!).

In the case of the final subject pronoun (it could be a full noun phrase too), the displacement puts the agent of the action into focus. “Lo prefiero comprar yo” has the old information first (the conversation is about buying pants) and then the news: it's not the other person who will buy the pants, but the speaker.

English cannot dislocate a pronoun in this way, so in the face of such a sentence, it has to resort to other means. With a simpler syntax it could be a cleft sentence, but here I suspect “It's me who prefers to buy them (the pants)” sounds terrible, and “The one who prefers to buy them is me” is ridiculous. The alternative is a reflexive pronoun added for emphasis: “I prefer to buy them myself”.

In Spanish it would be possible to use the emphatic mismo as well, but the pronoun would be dislocated anyway: “Lo prefiero comprar yo mismo”. If the speaker left it in the preverbal position it would sound exactly like English “I myself prefer to buy them”: the meaning would be different.