Collocations for informal - adj.

be
become
remain

extremely, fairly, very, etc.
relatively
Our meetings are relatively informal.

Informal is used with these nouns:
agreement, alliance, approach, arrangement, atmosphere, briefing, care, channel, chat, conversation, dinner, discussion, dress, economy, enquiry, feedback, gathering, instruction, interaction, language, learning, link, lunch, manner, meeting, network, poll, reception, recreation, register, rule, setting, survey, talk, training