If you're not sure which language to study, here are some factors to consider:
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.
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:
The languages with the most speakers are:
Here's an illustration showing where the most spoken languages are spoken.
See a larger version of this image at:
Each language presents you with a different set of challenges. Languages might have complex inflectional systems, complex writing systems, irregular spelling systems, and/or complex phonology. Generally the more a language differs from your L1 or other languages you know, the harder it is to learn.
For lesser-studied languages it can be hard to find language learning materials and courses. In some cases you might have to learn another language first, e.g. when learning indigenous languages of Latin America it helps if you know Spanish and/or Portuguese as most materials and courses are likely to be in those languages. Many languages are undocumented and have never been written, so training in field linguistics is needed in order to acquire them.
For English speakers the least difficult languages are probably: Italian, Spanish, French, Dutch, Afrikaans, German, Portuguese, Romanian, Norwegian, Swedish and Danish.
Arabic, Korean, Japanese and Chinese are often considered among the most challenging languages for English speakers, and speakers of other Western languages, to learn. Learning to read and write Chinese and Japanese is particulaly challenging, though the spoken languages are less difficult. Some of the indigenous languages of the Americas have complex grammar and phonology, for example Navajo verbs are all irregular.
Language Learning Difficulty for English speakers
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.
English, French, Swahili, Arabic, Portuguese
English, Spanish, Portuguese, French, Japanese
English, Japanese, Mandarin, Cantonese, Korean, Malay/Indonesian, Spanish, Portuguese
English, French, German, Spanish, Italian, Dutch, Flemish, Danish, Swedish, Norwegian, Portuguese, Finnish, Greek, Welsh, Russian, Japanese, Mandarin, Arabic
MLA Language Map Data Center - information based on census data about which languages are spoken in the USA: http://www.mla.org/map_data
Most common languages spoken in the U.S.
10 Best Languages to Learn Right Now (i.e. in 2010 in the USA)
BBC Voices - details of the languages spoken in the UK, with numbers