Break a leg
He's a few fries short of a Happy Meal
Listen to your heart
I smell a rat
Pulling my leg
Cat's got your tongue?
I feel like a million dollars
Jumping the gun
Kick the bucket
That car is a lemon
These common phrases mark our everyday speech. You know you're one of the locals if you understand what someone is really saying. These phrases are unique to the local culture however. Idioms are some of the toughest phrases to translate into another language. I remember doing some Greek translation and coming across some of the idioms used in the New Testament. Boy was I confused! Imagine though being from a different culture and coming to the U.S. and people start telling you to break a leg and just play it smooth. You'd think these people were out of their minds!
The thing is that if you don't know the local language, phrases, and idioms you'll always be somewhat of an outsider. Think about that. Knowing the idioms are like knowing the inside jokes. Someone on the outside just doesn't get it and there is a distance there. One issue with language training is that you may only learn the language of the classroom and not the real world. You'll never learn the idioms of the culture unless you 'stick your neck out' and live among the people!
If you ever become a missionary or just want to learn another language, do all you can to learn the idioms too. And learn them well so you aren't using them like an ill-timed joke that leaves everyone staring. I think that language learning is one part of the missionary life that can be neglected. With all the other responsibilities what does it matter that the language is spoken exactly as the locals do? Can't I get by without putting in all the extra work? Well, if you have ever sat through a 50 minute well intentioned sermon by someone whose first language was not English and their mastery of English wasn't that good....well, it will encourage you to learn the language the best you can if you ever go overseas! You don't want to put the people you are trying to reach through a near fingernails on the chalkboard experience every time you teach them because you haven't taken the time to master their language.
People will listen more closely and the gospel will seem less foreign if you can speak like a local and even use the local phrases and idioms. The writers of the New Testament did this. They used the local terms for coins, wages, places, and so forth. They worked hard to understand the cultures where they lived. They were following in the footsteps of their Master. Jesus used the local language. He dressed like the locals. He was the Incarnation - the God Man. Jesus didn't seem overly foreign to everyone He met. His message was the pure gospel which never changes. In the same way we need to bring the pure seed of the gospel to the cultures we reach and not bring our culture in addition to the gospel. But we need language to do this. Jesus brought the message of reconcilliation between a holy God and sinful man, in fact He was God but He looked, talked, and smelled just like a local - and we should too.