iPhone and Android apps show a list of channels with recent activity in your language or country. For channels you can understand update your Zello user profile with up to three languages that you speak. English language channels shown are those where most of the users are from your country.
Channels that are set to 18+ are not displayed unless you’ve changed settings. These are that have been flagged for excessive profanity, drugs or topics not appropriate for families.
To see 18+ channels -- Android: Options->Restrictions, iPhone: Setting->Restrictions
To flag a channel click the profile picture, then press the red flag.
See also Moderator Guidelines