Most Roman Catholic parishes are Regional Parishes. This means that their congregation comes from a specific geographic area. However, ethnic groups who communicate primarily or entirely in a longuage besides English creates a special pastoral need. In order to provide them with ministry in their primary language, there are also National Parishes. These parishes may be loosely defined by geography, but the area that their congregants come from will typically overlap several Regional Parishes, drawing all of the people who speak that language from those other areas. Additionally, other Nationaly Parishes dedicated to other languages will also overlap the same area.
This chart compares the growth of the Regional Parishes in the outer counties of the Philadelphia Archdiocese with the growth of the National Parishes. This graph clearly shows that nearly all of these parishes were founded in the couple of decades after about 1900.