---------------------------------------------------------------------------------------- Fomality level: same Meaning: almost same but slightly different. Friendliness: Both are unfriendly answers, because both don't answer the question directly. It is "嫌み（IYAMI)". ---------------------------------------------------------------------------------------- - The basic function of は is "emphasize". - なら　＝　に＋あら（未然形 of なり <be> ） = ならば　－(minus)　ば Aなら（A ならば）：if it is A, then.... ---------------------------------------------------------------------------------------- 魚が好きですか。-　The questioner exepects yes answer. 鶏肉は好きです。-　The answerer, emphasizing "鶏肉", implies "I don't like fish" 魚が好きですか。- The questioner exepects yes answer. 鶏肉なら好きです。 - The answerer, replying "If it is 鶏肉, I like it", strongly implies "I don't like fish"

There is no difference of being formal nor nice, nor depending on the situation. With my few knowlegde about English, I think the translation of the two answers are the same, "I like chicken", however, each one includes a amall difference of the feeling. 魚が好きですか。Do you like fish? 鶏肉は好きです。(No, I don't, but) I like chicken. This answer imagines us this person somply wants to say "I like chicken (comparing with fish)". 魚が好きですか。Do you like fish? 鶏肉なら好きです。(No, I don't, but) I like chicken. This answer imagines us this person wants to say "I like chicken (if I can talk/request about other food)". I hope this was helpful and please wait for better answers.