After English, I think that you may go for the French and Spanish as these two are also wide spoken and often get mixed with English so you may try even that too.
The intrinsic beauty of English is that it is a composite of many "external root languages". English was originally a Germanic tongue. Because of the Celtic influences, the Roman influences, and "loan words" brought into English during the renaissance period, spoken or written English can be grammatically "significantly wrong" whilst still carrying the meaning across, the point that one needs to convey to the recipient of information. Because English incorporates grammatical rules from several sources, there are "forms of sentence composition" which are analagous to many other unrelated tongues. That is a nice situation - or is it? To someone like myself, who appreciates the brevity of English and the precision needed to be effective in French, I resent the attitude of people who are content to see bad English that conveys the desired meaning. I love English because it is also the most precise and compact language of all. Go look at Gideon's bible. It does not matter what other languages are used, the English version is always the shortest.
The first is English. Now money are in Russia and Arabic Countries (also China). For me, the important is... 1º English 2º German 3º Spanish 4º French 5º Russian 6º Arabic 7º Mandarin 8º Portuguese 9º Italian 10º Other languages.
The 10 Most Useful Languages to Learn 10. Italian — Approximately 60 million speakers and the official language of the Vatican 9. Portuguese — Approximately 190 million speakers worldwide and spoken in over 20 countries 8. Japanese — Approximately 125 million speakers and the world’s 2nd-largest economy by GDP and 3rd-largest by purchasing power parity (PPP) 7. German — Approximately 120 million native speakers and a second language for many Europeans; it is the world’s 3rd-largest economy by GDP and 5th-largest by purchasing power parity (PPP) 6. Russian — Approximately 185 million native speakers and the second language for all nations of the former Soviet Union 5. Arabic (MSA) — Approximately 250 million speakers worldwide and the official language of 20 countries; it is the lingua franca of the Middle East 4. French — Along with English, French is the working language of the United Nations; it is also culturally and historically critical in Europe and much of North and West Africa 3. Mandarin Chinese – Approximately 880 million speakers, or 20% of the world’s population, and the second strongest economy in the world — after the United States — based on purchasing power parity (PPP) 2. Spanish — Approximately 350 million native speakers and the primary second language learned and spoken in the United States; Spanish is the language of trade and commerce in approximately 20 countries. 1. English — Most widely used language of business, commerce, and the internet and the primary second language learned in most countries