<?php

function fix_array(array $a)
{
    $result = [];

    foreach ($a as $char => $values) {
        foreach ($values as $lang => $value) {
            $result[$lang][$value] = $char;
        }
    }

    return $result;
}

$a = ['a' => ['en' => 2, 'fr' => 5], 'b' => ['en' => 4, 'mx' => 5]];
var_dump(fix_array($a));