One of the most common language games in English is Pig Latin, also known as Dirty Greek. Pig Latin is so wide-spread there are even various dialects. Some might say, "Histay isay Igpay Atinlay, oday ouyay understanday?" while someone else might say it, "Isthay isay Igpay Atlinlay, oday ouyay underayandstay?" Someone might say "ueenqay" and another could say "eenquay."
Here is a brief history of Pig Latin on The Straight Dope. Modern Pig Latin seems to have derived from Dog Latin, which is making modern English sound like Latin, or by translating English words to Latin without conjugation or declension. Manius peoplum do thus withoutium knowingus itus isum Historicalus. Vel, potius, id incendo similis hoc.
The most common method of Pig Latin is taking the first consonant or consonant cluster and bringing it to the end of the word and adding "ay" or "ey."
Happy --> Appyhay
Stay --> Aystay
Strong --> Ongstray
Sly --> Yslay
Occasionally, only the very first consonant is taken:
Stay --> Taysay
Strong --> Trongsay
Sly --> Lysay
Words that begin with a vowel do not carry anything. Usually, "ay" is added to the end when the word already ends in a consonant or consonant sound, and "hay," "yay" or "way" are added when the word ends in a vowel or vowel sound.
Ex. using "yay":
Up --> Upay
Use --> Useay [or] useyay
Herbs --> Erbshay [or] herbsay
Utopia --> Utopiayay [or] opiautay
Occasionally, "hay," "yay," or "way" are added regardless, other times it is only ever "ay" or "ey."
Ex. using "way":
Up --> Upway
Herbs --> Erbshay [or] herbsway
Compound words are usually split between the two words it's made from.
Understand --> Underayandstay
Schoolwork --> Oolschayorkway
Upside --> Upayidesay
Here are some online translators if you can't or are too lazy to do it yourself:
Pig Latin translator for your iPhone.
And, of course, Google in Pig Latin.