Distinguish English and Spanish with regular expressions
Any Language, 0.3677 (60.6064%, 1 byte)
a

No, I'm not joking. The single-character regular expression a successfully identifies Spanish words over English given your input files 60.6064% of the time, which makes it a valid submission.
Here's a complete, runnable Perl script that checks the percentage of this regular expression, assuming you've downloaded english.json and spanish.json into the same folder as the script.
#!/usr/bin/perl

use strict;
use warnings;
use 5.010;

my @english;
my @spanish;

my $fh;
open $fh, '&lt;', 'english.json';
while (&lt;$fh&gt;) {
    push @english, $1 if /&quot;(\w+)&quot;/;
}
close $fh;

open $fh, '&lt;', 'spanish.json';
while (&lt;$fh&gt;) {
    push @spanish, $1 if /&quot;(\w+)&quot;/;
}
close $fh;

my $correct = 0;
my $total = 0;

my $re = qr/a/;

for (@english) {
    $total++;
    $correct++ unless /$re/;
}
for (@spanish) {
    $total++;
    $correct++ if /$re/;
}

say &quot;$correct / $total (@{[100*$correct/$total]}%)&quot;;