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The question about the extent of the domination of the "English language on the "Internet has been historically, and is still, a controversial matter and in any case the relative representation of languages in the network is a fast changing data, although it is considered that amongst the more than 7000 existing languages less than 500(only 8.33% of total) have a digital existence as of today.
The two main indicators of languages on the Internet are the language of users of the Internet and the language of contents in the Internet.
The data about languages can be specified either as related only to "mother tongue (also referred to as first language and noted L1) or as related to first language plus "second language spoken (L1+L2). Data on second languages are far from being consensual and the differences are one of the main cause of discrepancy between data on languages used on the Internet.
As for the language of users, the main and most reliable source for persons connected to the Internet by country is the "ITU. From this "United Nation's authoritative source, two sources derive the persons connected by language, with some differences:
The differences between the figures seems to be related to the data about second languages and to the computing of the L1+L2 populations per language.
As for the language of contents, two sources exist, and they present important differences.
The FUNREDES/MAAYA Observatory argues that using Alexa rankings for the 10 millions sample of websites on which W3Tech applies a language recognition algorithm provokes a huge under-estimation of many Asiatic languages, primarily Chinese and languages from India. In the referenced paper and associated presentations, arguments are developed and warnings are made about the importance of "biases in the measurement of languages on the Internet.
There is debate over the most-used languages on the Internet. A 2009 UNESCO report monitoring the languages of websites for 12 years, from 1996 to 2008, found a steady year-on-year decline in the percentage of webpages in English, from 75 percent in 1998 to 45 percent in 2005. The authors found that English remained at 45 percent of content for 2005 to the end of the study but believe this was due to the bias of search engines indexing more English-language content rather than a true stabilization of the percentage of content in English online.
Ongoing monitoring by W3Techs showed that in March 2015, just over 55 percent of the most visited websites had English-language homepages. Other top languages that are used at least in 2 percent of the one million most visited websites according to W3Techs are "Russian, "German, "Japanese, "Spanish, "French, "Chinese, and "Portuguese.
The figures from the W3Techs study are based on the one million most visited websites (i.e., approximately 0.27 percent of all websites according to December 2011 figures) as ranked by "Alexa.com, and language is identified using only the home page of the sites in most cases (i.e., all of Wikipedia is based on the language detection of http://www.wikipedia.org). As a consequence, the figures show a significantly higher percentage for many languages (especially for English) as compared to the figures for all websites. The figures for all websites are unknown, but some sources estimate below 50 percent for English; see for instance, Towards a multilingual cyberspace  and the 2009 UNESCO report referenced earlier.
The number of non-English pages is rapidly expanding. The use of English online increased by around 281 percent from 2001 to 2011, a lower rate of growth than that of Spanish (743 percent), Chinese (1,277 percent), Russian (1,826 percent) or Arabic (2,501 percent) over the same period.
According to a 2000 study, the international auxiliary language "Esperanto ranked 40 out of all languages in search engine queries, also ranking 27 out of all languages that rely on the "Latin script.
All other languages are used in less than 0.1% of websites. Even including all languages, percentages may not sum to 100% because some websites contain multiple content languages.
Note that the Funredes/MAAYA Observatory offers quite different figures.
InternetWorldStats estimates of the number of Internet users by language as of June 30, 2016:
Note that the Funredes/MAAYA Observatory offers slightly different figures.