title: System locale error with fread
body: <p>I am trying to read a CSV using <code>data.table</code>'s <code>fread</code> where the decimal point is <code>,</code>. It returns an error about changing system locale. </p>
  
  <p>Any ideas how to proceed with that?</p>
  
  <pre><code>sampleCSV = "a;b;c
  1,03;80,3;50,4
  45,2;65,3;90,678"
  
  library(data.table)
  fread(sampleCSV, sep = ';', dec = ",", verbose = T)
  
  # *** Verbose &amp; Error message: ***
  # dec=',' but current locale ('C') has dec='.'. Attempting to change locale to one that has the desired decimal point.
  # Changing to system locale ('en_US.UTF-8') did not provide the desired dec. Now trying any provided in getOption('datatable.fread.dec.locale')
  # Trying 'fr_FR.utf8'
  # Sys.setlocale('LC_NUMERIC','fr_FR.utf8')  returned ""; i.e., this locale name is not valid on your system. It was provided by you in getOption("datatable.fread.dec.locale"). See ?Sys.setlocale and ?fread.
  # Error in fread(sampleCSV, sep = ";", dec = ",", verbose = T) : 
  #   Unable to change to a locale which provides the desired dec. You will need to add a valid locale name to getOption("datatable.fread.dec.locale"). See the long paragraph in ?fread.
  
  
  Sys.info()
  # sysname  release             version                                        machine
  # "Linux"  "4.4.0-28-generic" "#47-Ubuntu SMP Fri Jun 24 10:09:13 UTC 2016"  "x86_64"                                    
  
  sessionInfo() 
  # R version 3.2.3 (2015-12-10)
  # Platform: x86_64-pc-linux-gnu (64-bit)
  # Running under: Ubuntu 16.04 LTS
  # 
  # locale:
  #   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8   
  # [6] LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
  # [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
  # 
  # attached base packages:
  #   [1] stats     graphics  grDevices utils     datasets  methods   base     
  # 
  # other attached packages:
  #   [1] data.table_1.9.6
  # 
  # loaded via a namespace (and not attached):
  #   [1] tools_3.2.3  chron_2.3-47
  </code></pre>