The current pluralization rule for French is not exactly right. Numbers like negatives (like -4) and decimals of 1 (like 1.78) should also be considered as singulars, not plurals. Correct me if I'm wrong, but the current rule sets only 0, decimals of 0, and 1, as singulars, and the rest as plurals.

Something like that is better:

return [
    if ($number >= 0 and $number < 2) {
        return 'singular';
    }
    else {
        return 'plural';
    }
]

(Sorry, I'm not a coder, so I can't really provide the appropriate patch myself)