Is there any other short cut method to change the language, for example when you are writing an email? It is a loss of time to put on hold your email, go in Settings, then Generall, Keyboard, International...

No,Not if it benefits you to accomplish what you are trying to do.

You can get keyboard for other languages and have the always available.

Settings > General > Keyboard > Keyboards > Add New Keyboard > Select the Keyboard you wish to add.

Then when typing your email you will notice that a key with a Globe has appeared in the lower row of keys to the left of the space bar. Tapping the Globe key will switch the keyboard to the second keyboard. Tap the Globe key again to return to the original keyboard.