How is it possible that Gollum speaks Westron?

Question

In the "Concerning Hobbits" section of the Prologue to The Lord of the Rings, after the explanation of how the three breeds of Hobbits gradually moved to Eriador it says

“in those days also they forgot whatever languages they had used before, and spoke ever after the Common Speech, the Westron as it was named....”

The Prologue seems to say that Hobbits switched to Westron after moving to Eriador. Sméagol, a Stoor who lived near the Gladden Fields, did not cross the mountains into Eriador, but fled underground and lived in isolation for 500 years. If he abandoned society before it began to use the Common Speech, how is it possible that he speaks it (albeit in his oddly sibilant variant)? It’s possible that I’m misinterpreting the text and that though Hobbits abandoned other languages after Sméagol’s time in society, they were already aware of Westron before moving to Eriador. This to me seems unlikely: Sméagol’s community was isolated and probably had little contact with Men. What gives?

Answer 1

The Stoors lived in Eriador before Smeagol's people moved to the Gladden.

Firstly, the Hobbits had began to learn letters and languages long before Smeagol was even born, let alone before he left society. Moving on from that, it is suggested in the appendices that the Stoors had lived in Eriador before some returned to the Wilderlands.

In Appendix A we're given a bit more insight into the movements of the Stoors, and it would appear that earlier in the Third Age, they had lived in Eriador but were forced to flee due to the wars of Angmar. This would suggest that the Stoors had dealings with Men at the time, being part of the Kingdom of Arnor, allowing them to have been taught letters and languages.

It was at this time that the Stoors that had dwelt in the Angle (between Hoarwell and Loudwater) fled west and south, because of the wars, and the dread of Angmar, and because the land and clime of Eriador, especially in the east, worsened and became unfriendly. Some returned to Wilderland, and dwelt beside the Gladden, becoming a riverside people of fishers.
_{The Return of the King: Appendix A, "Eriador, Arnor and the Heirs of Isildur}

(It is worth noting, that the Angle spoken about above is just West of Rivendell as seen in the map linked here, between the rivers Loudwater and Hoarwell)

Later in Appendix B it again suggests that the Stoors moved from Eriador back over the Misty Mountains, and therefore most likely learnt the Common Tongue before moving back over.

1356 King Argeleb I slain in battle with Rhudaur. About this time the Stoors leave the Angle, and some return to Wilderland.
_{The Return of the King: Appendix B, "The Third Age"}

It would suggest to me that the phrasing in the prologue is not as clear as it could be, and the Appendices paint a far clearer picture that the Stoors came over the Mountains into Eriador and then split with some returning to the Wildlands. Furthermore, you seem to be confusing how old Smeagol is with how long ago these travels were taking place. The death of King Argeleb took place 1000 years before Smeagol was born.

Answer 2

The language of his original clan was five centuries removed from the language of the Shire hobbits, so if he had remained in his native dialect communication with Bilbo would have been difficult at best.

Although his idiosyncrasies of speech are in a large way due to the Ring eating his brain, Gollum spent a lot of time listening in to the orcs with whom he shared that cave for so long. Orcs tend to adopt whatever language is common in their area, and thus Gollumn maintained some fluency in the language of the area.