i18n: replace Javascript with JavaScript

[shirayuki]

Replace Javascript with JavaScript for consistency with other messages

[codecov-commenter]

Codecov Report

Merging #715 (54e9da6) into master (83d17b7) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #715   +/-   ##
=========================================
  Coverage     39.31%   39.31%           
  Complexity     2046     2046           
=========================================
  Files            75       75           
  Lines          7233     7233           
=========================================
  Hits           2844     2844           
  Misses         4389     4389           

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[kghbln]

Thanks!