Languages with sub dialects such as Common and Primordial are able to understand the other languages if there is context, or enough time to process the information.
Common
Dialects:
Dwarvish
Elvish
Giant
Gnomish
Goblin
Halfling
Orcish
Abyssal
Celestial
Draconic
Deep Speech
Infernal
Primordial
Dialects:
Sylvan
Undercommon
Druidic (Druids)
Thieves’ Cant (Rogues)