All of the Japanese I'm currently studying is the polite form of verbs. Your standard Japanese dialect has two forms - standard and polite. The polite form is considered more appropriate in public forums, the standard form among friends and family. Therefore anyone in Japan can get along reasonably well knowing only one of the two - same meaning, different presentation.
In addition to the declining use of Kanji, especially among the younger generation in Japan, Japan is starting to see one of its most honorific languages wither away - Keigo. Keigo (敬語) is used especially to signify differences in social classes. This can be a little confusing so let me give you an example.
Standard suru Polite shimas Sonkeigo nasaimasu (exaulted, referring to someone else) Kenjōgo shiteorimasu (humble, referring to yourself or your equals)
As you can see just by the different conjugations of the one verb, Keigo is like another language in itself. Fifty years ago, it's use was common and accepted among Japanese citizens. Today, however, the younger crowd just isn't seeing the point. Although Japan is still one of the most polite languages on the planet, it is slowly being broken down to a single form. Young people in Japan just don't place as much emphasis on honorific speech as their parents and grandparents did. It won't be long at this rate...
Although Keigo may not be long-lived, the Japanese language continues to flourish and divide, divide, divide. Case in point - the different dialects of Japan. Each major region has their own Japanese dialect, just as you'd expect from any language over a large enough landmass - probably even more pronouced in Japan due to the natural topography travelers had to overcome to visit cities; as a result, the original language developed independently in different cities and new dialects sprang into existence.
The most difficult of these to understand, even for other Japanese people, is Osaka-Ben or Kansai-ben: the dialect spoken in the Kansai area. Think of it as like really developed slang - the standard Japanese dialect can still be understood and used, but it's more common to speak with the regional dialect.