Problem with latest version of babel

Question

I have a problem with that latest version of babel (3.10). If I compile this file:

\documentclass{article}
\usepackage[ukenglish]{babel}
\pagestyle{empty}
\begin{document}
Hello world!
\end{document}

Then I get the message

LaTeX Error: Command \setlocale already defined.

Why is that?

Here is my log file (this time, I used spanish instead of ukenglish):

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (MiKTeX 2.9.6350 64-bit) (preloaded format=pdflatex 2017.5.8)  29 MAY 2017 11:13
entering extended mode
**./test.tex
(test.tex
LaTeX2e <2017-04-15>
Babel <3.9t> and hyphenation patterns for 74 language(s) loaded.
("C:\Program Files\MiKTeX 2.9\tex\latex\base\article.cls"
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
("C:\Program Files\MiKTeX 2.9\tex\latex\base\size10.clo"
File: size10.clo 2014/09/29 v1.4h Standard LaTeX file (size option)
)
\c@part=\count79
\c@section=\count80
\c@subsection=\count81
\c@subsubsection=\count82
\c@paragraph=\count83
\c@subparagraph=\count84
\c@figure=\count85
\c@table=\count86
\abovecaptionskip=\skip41
\belowcaptionskip=\skip42
\bibindent=\dimen102
)
("C:\Program Files\MiKTeX 2.9\tex\generic\babel\babel.sty"
Package: babel 2017/05/19 3.10 The Babel package

("C:\Program Files\MiKTeX 2.9\tex\generic\babel\switch.def"
File: switch.def 2017/05/19 3.10 Babel switching mechanism


! LaTeX Error: Command \setlocale already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

\GenericError  ...                                
                                                  \endgroup 
l.301     {Find an armchair, sit down and wait}}

? 
)
*************************************
* Local config file bblopts.cfg used
*
("C:\Program Files\MiKTeX 2.9\tex\latex\arabi\bblopts.cfg"
File: bblopts.cfg 2005/09/08 v0.1 add Arabic and Farsi to "declared" options of
 babel
)
("C:\Program Files\MiKTeX 2.9\tex\latex\babel-spanish\spanish.ldf"
Language: spanish.ldf 2016/03/03 v5.0p Spanish support from the babel system

("C:\Program Files\MiKTeX 2.9\tex\generic\babel\babel.def"
File: babel.def 2017/05/19 3.10 Babel common definitions
\babel@savecnt=\count87
\U@D=\dimen103
)
\es@quottoks=\toks14
\es@quotdepth=\count88
Package babel Info: Making " an active character on input line 561.
Package babel Info: Making . an active character on input line 662.
Package babel Info: Making < an active character on input line 707.
Package babel Info: Making > an active character on input line 707.
)) (test.aux
LaTeX Info: Redefining \. on input line 6.
LaTeX Info: Redefining \% on input line 6.
)
\openout1 = `test.aux'.

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 6.
LaTeX Font Info:    ... okay on input line 6.
LaTeX Info: Redefining \. on input line 6.
LaTeX Info: Redefining \% on input line 6.

[1

{C:/Users/User/AppData/Local/MiKTeX/2.9/pdftex/config/pdftex.map}] (test.aux
LaTeX Info: Redefining \. on input line 6.
LaTeX Info: Redefining \% on input line 6.
)

 *File List*
 article.cls    2014/09/29 v1.4h Standard LaTeX document class
  size10.clo    2014/09/29 v1.4h Standard LaTeX file (size option)
   babel.sty    2017/05/19 3.10 The Babel package
 bblopts.cfg    2005/09/08 v0.1 add Arabic and Farsi to "declared" options of b
abel
 spanish.ldf
 ***********

 ) 
Here is how much of TeX's memory you used:
 1053 strings out of 493330
 13040 string characters out of 3139261
 74021 words of memory out of 3000000
 4656 multiletter control sequences out of 15000+200000
 3640 words of font info for 14 fonts, out of 3000000 for 9000
 1141 hyphenation exceptions out of 8191
 27i,4n,30p,169b,359s stack positions out of 5000i,500n,10000p,200000b,50000s
<C:/Program Files/MiKTeX 2.9/fonts/type1/public/amsfonts/cm/cmr10.pfb>
Output written on test.pdf (1 page, 12297 bytes).
PDF statistics:
 10 PDF objects out of 1000 (max. 8388607)
 0 named destinations out of 1000 (max. 500000)
 1 words of extra memory for PDF output out of 10000 (max. 10000000)

Answer 1

There is no error with babel version 3.12.

However, you should change ukenglish into UKenglish. The ukenglish option has never existed, but due to the filesystem in Windows being case insensitive, the correct file UKenglish.ldf is loaded anyway under that OS.