If the language pack contains UTF-8 encoded non breaking spaces, some strings display wrong.
To reproduce, follow these steps:
1) Switch to the french language.
2) Log in and see "Connecté sous le nom « Your Name Here »" in the top right of the window (correct behaviour).
3) Log out
4) In the file moodle.php of the french language pack, replace the in the string $string['loggedinas'] with a UTF8 encoded non breaking space.
5) Log in and see the "Connecté sous le nom «" (nothing more) in the top right of the window (bug).
The correction of this bug could be very handful, as it will allow a better lisibility of the lang pack and the consistent use of non-breaking spaces in all strings (i.e also those that are sent by standard mail, which doesn't support HTML entities).