Considering that French and English are the two official languages of the EU, you don't really need French if you know English. French is only spoken in France, and to degree in some countries in Africa. Basically, it's not worth the hassle of learning.
Castilian, usually known as Spanish, which is spoken in all the national territory, Equatorial Guinea, the Sahara, Central and South America ,except Brazil and the Guyanas, and parts of the United States and the Philippines, is the second language in the world in terms of numbers of speakers ,over 330 million.
Mandarin is the most widely spoken of all Chinese languages. It is used by 867 million people as a first language in a vast Beijingarea of northern and southwestern mainland China. It is also spoken in Brunei, Cambodia, Indonesia, Laos, Malaysia, Mauritius, Mongolia, Philippines, Russia, Singapore, Taiwan, Thailand, United Kingdom, USA, and Viet Nam. The total number of first-language speakers of Mandarin world-wide is estimated to be around 873 million. In addition, it is used as a second language by another 178 million people
Over 309 million people speak English as their first language, as of 2005. English today is probably the third largest language by number of native speakers, after Mandarin Chinese and Spanish. However, when combining native and non-native speakers it is probably the most commonly spoken language in the world, though possibly second to a combination of the Chinese Languages, depending on whether or not distinctions in the latter are classified as "languages" or "dialects." Estimates that include second language speakers vary greatly from 470 million to over a billion depending on how literacy or mastery is defined. There are some who claim that non-native speakers now outnumber native speakers by a ratio of 3 to 1.
To sum it all up :
The Summer Institute for Linguistics (SIL) Ethnologue Survey (1999) lists the following as the top languages by population:
(number of native speakers in parentheses)
1. Chinese* (937,132,000)
2. Spanish (332,000,000)
3. English (322,000,000)
4. Bengali (189,000,000)
5. Hindi/Urdu (182,000,000)
6. Arabic* (174,950,000)
7. Portuguese (170,000,000)
8. Russian (170,000,000)
9. Japanese (125,000,000)
10. German (98,000,000)
11. French* (79,572,000)
Meaning that if you know Mandarin, Spanish and English you can go basically anywhere in the world and not have problems with the language. You only need those 3 languages, all other languages are sh!t.