Availability
Are materials and other resources available for the language you want to learn? Are there classes in your area? For the popular languages, like French, Spanish and German, this shouldn't be a issue, but it may be difficult to find resources and/or classes for the lesser-studied languages.

Usefulness
If you want to learn a language with a large number of speakers and which is spoken in many countries, the ones to choose in order of 'usefulness' are: English, French, Spanish, Russian, Arabic, Chinese (Mandarin), German, Japanese, Portuguese and Hindi/Urdu

This list is based on the number of speakers, the number and population of countries where the languages are spoken, the number of major fields using the languages internationally, the economic power of countries using the languages, and their socio-literary prestige.

For a breakdown of these factors see:
http://www2.ignatius.edu/faculty/turner/languages.htm

Spanish is the most widely spoken language in the USA after English, with around 28 million speakers. The third most spoken language is Chinese - mainly Cantonese, with over 2 million speakers. Other languages with over a million speakers include French, German, Tagalog, Vietnamese and Italian.

For more details see: http://www.mla.org

Difficulty
Each language presents you with a different set of challenges. Some languages have numerous verb conjugations, noun declensions and/or genders. Others might have irregular spelling systems, a variety of plural forms and/or uncommon phonemes or combinations of phonemes. Generally the more a language differs from your L1 or other languages you know, the harder it is to learn.

For English speakers the least difficult languages are probably: Italian, Spanish, French, Dutch, Afrikaans, German, Portuguese, Romanian, Norwegian, Swedish and Danish.

The most difficult languages for English speakers, and indeed speakers of most other languages, are Arabic, Korean, Japanese and Chinese.

Language Learning Difficulty for English speakers
http://www.nvtc.gov/lotw/months/november/learningExpectations.html

Languages in demand by employers
If you want to learn a language in order to improve your employment/promotion prospects then choose one that is in demand by employers. The list below gives you an idea of which languages are in demand and is based on job ads posted on recruitment sites. You can find links to the recruitment sites on the Careers using languages page.

Note: languages are shown more or less in order of popularity with employers.

Africa
English, French, Swahili, Arabic, Portuguese

Americas
English, Spanish, Portuguese, French, Japanese

Asia-Pacific
English, Japanese, Mandarin, Cantonese, Korean, Malay/Indonesian, Spanish, Portuguese

Western Europe
English, French, German, Spanish, Italian, Dutch, Flemish, Danish, Swedish, Norwegian, Portuguese, Finnish, Greek, Welsh, Russian, Japanese, Mandarin, Arabic

• Working memory.
• Cognitive Load Theory
• Что такое "разговорный английский"
• Ten tips to build English vocabulary
• Если вы думаете, что транскрипция вам ни к чему :-)