regardless of the actual system locale addlocale will always show "en_US" because this is the default for the system reset. I don't understand why you get the error message, you shouldn't, and this usually indicates either an incomplete addlocale run, or running addlocale on a borked system. Do you get these messages also when you open a terminal? On the other hand, if all works fine, then you could just ignore these boot-up messages.
But what exactly means addlocale seemed to install ok
? Seems to me that you maybe started again from an incomplete addlocale run? Ibiblio seems to work fine again now, so please ensure to
1) have a functional Synaptic setup, see 1
, and/or 2
, then and only with a functional Synaptic setup
2) run addlocale and reset to en_US (in order to restore the default system setup)
3) run addlocale and choose en_GB
this should work fine and not provide any error messages.