1. **INTERNATIONAL**
2. **LANGUAGE**
3. **SPEAK**
4. **DIFFICULT**
5. **MILLION**
6. **IT**
7. **CENTURY**
8. **LANGUAGE**
9. **SPEAK**
10. **FOR**
So the completed text would read:
English is the (1) **INTERNATIONAL** language of the air. All pilots must speak English so that other pilots and air traffic controllers can understand them. English is also a very useful language (2) **LANGUAGE** for the passengers. All check-in clerks must speak English, and all announcements on planes (3) **SPEAK** given in English.
Today, when English is one of the major languages in the world, it’s (4) **DIFFICULT** to realize that this is a recent thing – that in Shakespeare’s time, only a few (5) **MILLION** people spoke English. English has become a world language because (6) **IT** is established as a mother tongue outside England, in all the continents of the world. This exporting of English began in the seventeenth (7) **CENTURY**, with the first settlements in North Africa. Above all, it is the great growth of population in the United States that has given the English (8) **LANGUAGE** its present standing in the world.
People who (9) **SPEAK** English fall into one of three groups: those who are learned it as their mother tongue, those who have learned it as a second language, and those who are forced to (10) **FOR** professional or educational purpose.