Keep translation files in sync?
You could manage all your translations in one csv file. There's another Q/A about translation files, where @Bill Criswell explains how you can use csv files for translations. I added some variables to his script to make it well adaptable to multiple locales.

Set up your new csv file with a column for each locale. Then add the script to each of your lang.php files and configure $this_locale_column according to the file's locale.

&lt;?php

$path = $_SERVER['DOCUMENT_ROOT'].'/../craft/translations/translations.csv';
$delimiter = ';';
$skip_rows = 1; //skip header rows
$primary_locale_column = 0; //set primary locale column
$this_locale_column = 1; //set secondary locale column for this `lang.php` file

$translations = array();
$translations_file = fopen($path, 'r');

if ($translations_file) {
    while(($translation = fgetcsv($translations_file, 1024, $delimiter)) !== FALSE) {
        if($skip_rows != 0) { $skip_rows--; continue; }
        $translations[$translation[$primary_locale_column]] = $translation[$this_locale_column];
    }
}

return $translations;