Hello,
If in Book library - you have e.g. 2 languages, and you installed to Joomla these 2 languages, if when you switch in yours site Backend these languages (without Joomla fish) - Book library change self language interface - for you in Book library all good, so you have some problems with Joomla Fish. If in book library language not change - problem in Book Library. So please write tu us again
Thanks,
OrdaSoft team