The CIA Word Factbook has a field listing for languages spoken per country and percentage of population that speaks it.
https://www.cia.gov/library/publications/the-world-factbook/fields/2098.html
For finer grain, you can Google 'language spoken home dataset'. This will find census datasets related to the distribution of languages spoken. Below are some examples:
United States (Census) by state:
https://www.census.gov/hhes/socdemo/language/data/other/detailed-lang-tables.xls
City of Chicago, IL by neighborhood:
https://data.cityofchicago.org/Health-Human-Services/Census-Data-Languages-spoken-in-Chicago-2008-2012/a2fk-ec6q
City of Cambridge, MA by neighborhood:
https://data.cambridgema.gov/Neighborhood-Census-Data/2007-2011-Language-Spoken-at-Home-by-Neighborhood/sba5-5kgg
Madison County, NY
http://cnyvitals.org/madison/demographics/language-spoken-at-home/
State of Hawaii:
https://data.hawaii.gov/Culture-and-Recreation/English-ability-by-language-spoken-at-home/i9hq-hna6
Canada (from their Census) by Province, Electoral District:
http://data.gc.ca/data/en/dataset/81a2bd6e-622f-4f17-84c1-215216485992
Queensland, AU by statistical area:
https://data.qld.gov.au/dataset/language-spoken-home-sa4-qld/resource/9fa1fc3a-ab09-4c99-a60f-f95c59269492
Greater London by borough:
http://publicdata.eu/dataset/first-language-spoken-at-home-borough0e333
UNESCO Atlas of the World's Languages in Danger:
http://www.unesco.org/culture/languages-atlas/