Yamaguchi Prefecture has a relatively long history, as it is located close to China. There are no big cities, but there are many attractive and interesting smaller cities, where traditional Japanese rural landscapes can still be seen on the outskirts.
People speak Yamaguchi Dialect, which is very similar to the dialect that is spoken in Tokyo, but the vocabulary is somewhat different.
When you get in Yamaguchi prefecture, you may see the sign of "Oidemase（おいでませ）" - meaning "Yokoso" or "Welcome" in Yamaguchi Japanese.
In Iwakuni, there is an American base, so some people understand English well. But in other areas, especially in rural areas, few people understand English well. So learning some basic Japanese phrases will help you so much.
In Shimonoseki, there are many Koreans. You will find some signs and guides written in Korean. You might also find people who speak Korean if you try.
Yamaguchi Ube Airport(UBJ), the prefectural airport is in Ube city. Yamaguchi Ube Airport regularly connects to Haneda Airport in Tokyo, Japan. Both JAL and ANA are offering the flights from Haneda to Ube. It's about 90 minutes' flight from Haneda to Ube.