Um, Sinnoh is totally based off of the northern most island of Japan, which is called Hokkaido. The map is even just the island rotated a bit. Just look at this: http://cdn.bulbagarden.net/media/upload/a/a2/Pokemonworld.png (it even shows what parts of Japan other regions are based off of)

The only region for the core games that is loosely based off of a real life location is Unova, which was inspired by America.

@Void: This ended up being really, really long, so if you (or the other people that decided to read this post) don't feel like reading it all, just jump to the two separate TL;DRs.

For Johto the city/town names come from: New Bark – tree (since it's just literally new bark), Cherrygrove – a grove of cherry trees, Violet – flower and a color, Azalea – flower, Goldenrod – flower and a color, Ecruteak – combination of a color and a tree, Olivine – combination of a color (olive, which is also is a fruit that grows on trees) and vine, Cianwood – combination of a color (cyan) and wood, Mahogany – a group of different kinds of wood known for a reddish brown color, Blackthorn – the name of a shrub/small tree.

The legendary beasts (Raikou, Entei and Suicune) are based off of the three events that happened to the Brass Tower (aka the Burned Tower). Raikou – the lightning that struck it, Entei – the fire that burned it, Suicune – the rain that put out the fire.

TL;DR – Johto has names all come from plants, with some being combined with colors (probably in reference to Kanto). The legendary beasts are only connected with the plot.

For Sinnoh the city/town names come from: Twinleaf – bud/sprout/set of two leaves, Sandgem – sand + gem, Jubilife – jubilant + life, Oreburgh – ore + the suffix "burgh" meaning city, Floaroma – flora + aroma, Eterna – shortening of eternal, Hearthome – from the expression "home is where the heart is", Solaceon – from "solace" meaning comfort, Veilstone – veil + stone, Pastoria – from "pasture land", Celestic – from celestial, Canalave – canal + lave, Snowpoint – snow + point, Sunyshore – sunny + shore.

Japanese Sinnoh names (using the English names of the cities/towns so they can be recognized): Twinleaf – bud/sprout/set of two leaves, Sandgem – sand, Jubilife – a Japanese expression similar to "long life!" and "congratulations!", Oreburgh – iron, Floaroma – garden, Eterna – eternal, Hearthome – "a way of life", Solaceon – carefree or easy, Veilstone – curtain, Pastoria – field or outdoor, Celestic – medium or shire maiden, Canalave – waterway, Snowpoint – the name of the tip of a katana, Sunyshore – shore.

There's two trios in Sinnoh – the creation trio (Diagla, Palkia, Giratina) and the lake trio (Mespirit, Uxie, Azelf). Both groups were created by Arceus and it's worth mentioning that the two groups do interact with each other (which can be seen in the games). Diagla represents time, Palkia is space, and Giratina is antimatter. Mespirit is the being of emotion, Uxie is knowledge, and Azelf is willpower.