How is Bard able to understand the thrush’s message (understand its tongue)?

Bard is able to understand the thrush because Bald is the descendant of Girion of Dale , they have the ability to understand thrush which make Bard to be able to understand the thrush.

Explanation:

The Hobbit is the story of Bilbo Baggins, a hobbit who lives in Hobbiton. He was a lover of comfort and seek pastoral life but he later became a fighter after he visited wizard Gandalf. They embark on a journey to find the lost treasure guarded by dragon Smaug. Bard is an excellent warrior from Esgaroth town. the grim-voiced, grim-faced man, is willing to challenge Snaug and he has help from a messenger bird, called a thrush. The thrush relays information that Bilbo discovered while in Smaug's lair and he killed smaug. Bard is able to understand the thrush because Bald is the descendant of Girion of Dale which make Bard to be able to understand the thrush.

no