Dear,

I just installed MLF in French and precisely had the same issue. I confirm that placing [default] as you advised makes it work.

Please note that this solves another strange thing : the lang file encodes accentuated letters with symbols, but the result was not always good in the browser.

For instance :
reply_link = Répondre à ce message (for the French Répondre à ce message)
BUT
register_pw_conf = Répéter le mot de passe:
=> I had begun to correct the lang file like that because otherwise the symbols would appear in the browser instead of the accents.

But now, by adding [default], I realise that the browser needs the words with symbols.
So I have to change the fields back hat I had already corrected...

Now another question: does it really takes a day to activate the account as the confirmation message says ???

Cheers,
Jean-Michel