Invoice title translation in .po file does not have effect on generated pdf (always says Invoice).
Something not right here:
./plugins/invoice/form/invoice.php on line 79

<?php echo ($inv['s_title'] <> '' ? $inv['s_title'] : __('Invoice', 'invoice')); ?>

Invoice is just default string, once you save it it is stored.

I am sorry but I am confused.
'INVOICE' lang phrase printed in top right corner of the invoice is in messages.po and there is a hook to the code I quoted.
But translation doesnt work because code form line 79 does not read properly this phrase translation from .po file.

I am not sure what you mean saying once you save it it is stored?
 

@piter
Once you save it, it saves what you put there no matter what language will be used.

@MBT, please check my previous message because I just edited it when you posted your reply.
Still, not sure what you meant Once you save it, it saves what you put there. Yes 'there' - you mean where?? It is in .po file :-)

@piter
Into invoice.

Oh I found it now but purely by random,
Please add some frame around this textarea as with other editable elements.
Otherwise it's not so obvious that this phrase can be edited ... :-)
Thanks