School kids, most tour guides, and younger people on Dominica speak English with a delightful lilt and occasional French pronunciation of some words. We could talk easily with them, and there was no trouble understanding what was being said. However, your best bet for communicating with older folks is to speak slowly and, if necessary, write down what you want to say. We thought we understood one local grandmother when we asked for directions to the Queen Mary Street entrance to the Botanical Gardens. She readily offered to take us to the street, and we cheerfully tagged along behind her and then found she had taken us to her church, which was adjacent to the gardens. Because she was so happy to have visitors in her church, we ended up staying for a rousing service with great singing and rhythmic clapping and foot-stamping. This was a misunderstanding that worked out well, but if we had had to be back at the airport or the docks by a specific time, this kind of confusion could have been a real problem.
A French-English dictionary is not much help because the local French dialect has unique words and phrases. One highly literate young teacher explained to us that much of the local dialect is hundreds of years old, and if compared with English, is very nearly Shakespearian in vocabulary and structure.