Here are the completed sentences based on the information provided in the document:
1. A "lingua franca" is a language used as a common language between speakers whose native languages are different.
2. A "speech community" is a group that includes all the speakers of a single language.
3. "Endangered" means threatened with extinction or disappearance.
4. A "global language" is an international language used for communication between speakers of different native languages which spreads widely around the world.
5. "Indigenous" means originating in and typical of a particular region or country.
6. A negative aspect of using English as a global language is that it can threaten local cultures and languages