These characters learned to communicate in an insular and limited fashion of a typically localized (slum, area, community, etc.) birth or dominant culture. Their primary language is spoken with a thick accent and localized style (-1 on effective level of communication; including Charisma based abilities and social interaction with those not part of the locale). Abilities requiring literary (incl. encryption/cipher) and verbal fluency are all penalized, while understanding others is not.
Even a character possessing rudimentary skills can use them to persuade, deceive, etc. within the context of what they can express, because they have the basic social cues and context to make their skills apply. A challenged/poor communicator lacks this, and/or has a speech pathology or grasp of the language structure that is completely lost on the communicator and actually inhibits communication.