English check OK:
abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon angle
French check OK:
abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon brave
both check OK:
abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon amateur double

Here are 3 phrases, all using only the common words of English and French.

One only checks OK on the English wordlist, one only checks OK on the French wordlist, and the last one checks OK on both lists.

Entering all three into Copay using the following locale settings will yield the following results:

'en':

1. Pass
2. Fail
3. Pass

'fr':

1. Pass
2. Fail
3. Pass

The expected behavior would be:

'en':

1. Pass
2. Fail
3. Pass

'fr':

1. Fail
2. Pass
3. Pass

Suggestion:
Depending on the locale of Copay, pass a wordlist to bitcore-mnemonic in this line:
https://github.com/bitpay/bitcore-wallet-client/blob/b342e6d8f9be23a904dbff3219028bc47a47e322/lib/credentials.js#L86
like this: var m = new Mnemonic(words, localewordlist);

There is a possibility (though very small) that a French phrase generated by Copay can not be recovered in Copay due to this. (Unless you are checking the mnemonic output again using Mnemonic.isValid() after generating.