Suppose you add a new field named 'language'. You will find some oddities on your redirect page after submitting the form that this field is in. This is because 'language' is already an internal field which contains the language of the viewer of the page, and so there's a conflict between two variables with the same name.

If you encounter a problem after submitting a form, consider a naming conflict as a possible soure of the problem. Try renaming your field to something more unique (such as myspeciallanguage) to ensure that there are no conflicts.

Do not bother worrying about this if you have not encountered a problem -- the odds that you will happen to choose a name that conflicts are small.