こんな寒い中待たせてごめんね！って英語でなんて言うの？

"I am sorry to keep you waiting in the cold!" This is one of the simplest forms of expressing this idea. However, you could also say.... "It is cold out here!I am sorry to have kept you waiting in this weather!" (In this sentence you first acknowledge that it is cold outside and then apologize for keeping them waiting.) Hope this helps.

The best way is to be straight forward. Apologise for keeping them waiting in the cold. Sometimes it can't be helped and most people will understand. "I aplogise for keeping you waiting outside", can let them know that it was an accident and you are apologetic. "Sorry I am late, you must be freezing, quick come in!" Your friend or colleague will appreciate your apology and be glad to get out of the cold.

"I apologize to keep you waiting in the cold." expresses you remorse at keeping a person waiting for a time in the cold.

Well, waiting in a cold place for an unreliable friend can be a rather uncomfortable experience. Maybe better to meet in a warm cafe next time?

To 'apologise' is to also mean you are 'saying sorry' so you could say 'I'm sorry to keep you waiting' or 'I apologise for keeping you waiting' by adding 'in the cold' means that they are waiting for you in the cold weather

Sorry for making you wait. Sorry I was late. Sorry you had to wait outside in the cold. Are suitable ways of apologising for making someone wait in the cold.

When greeting or speaking to someone who is a friend, informality is a sign of affection. If you are enthusiastic in your apology it will be well received.