If I could speak English fluently.

If I could speak English fluently, I would explain our products in our shop to to foreign customers. 
Then, I would ask them about their country their trip, and their impression of Japan.
Right now I can't understand them, so I want to be able to talk with them in English.

If I could speak English fluently, it would make trips abroad much easier for me. I would be able to communicate with people at airports, hotels, restaurants, and at shops. 

If I could speak English fluently, I would teach(?)Japanese to foreigners in English.

If I could speak English fluently, then I would have many more opportunities. I will work harder to become fluent in Japanese. 

Pretty good!  But you should review English conditionals. ^^

If I can could speak English fluently

If I can could speak English fluently, I would explain to foreign customers about our products in our shop in English.
Then I would want to ask them about their country and their trip, also their impression of Japan.
Now Currently, I can't understand their English. I want to talk with them in English.

If I can could speak English fluently, it would have been more is convenient for trips abroad; trip, at the airport, hotel, restaurants and shops.

If I can could speak English fluently, I want to tell would talk about Japanese culture to foreigners in English.

If I can could speak English fluently, the I would have had more opportunities possibilities spread. I will work harder to enhance my English language skills.