You can NOT reference column aliases in the SELECT clause within the WHERE clause, at least not in the same query. You could subquery it, or just use the original column references.
select
bobooks."ID" request,
bobooks."TITLE",
bobooks."AUTHOR",
bogenres."NAME" genre,
bobooks."OWNER",
bostatus."NAME" status,
bolanguages."LANGUAGE" language,
bolanguages2."LANGUAGE" secondary_language
from BO_BOOKS bobooks
inner join
BO_GENRES bogenres on bobooks.genre = bogenres.id
inner join
BO_STATUS bostatus on bobooks.status = bostatus.id
inner join
BO_LANGUAGES bolanguages on bobooks.language = bolanguages.id
left outer join
BO_LANGUAGES bolanguages2 on bobooks.secondary_language = bolanguages2.id
where
(replace(:P19_AUTHOR, ' ', '') = ''
or
bobooks.author like '%'||:P19_AUTHOR||'%')
AND
(replace(:P19_TITLE, ' ', '') = ''
or
bobooks.title like '%'||:P19_TITLE||'%')
AND
(:P14_LANGUAGE = 'all'
or
bolanguages."LANGUAGE" = :P19_LANGUAGE
or
bolanguages2."LANGUAGE" = :P19_LANGUAGE)
AND
(:P19_GENRE = 'all'
or
bogenres."NAME" = :P19_GENRE)
AND
(replace(:P19_OWNER, ' ', '') = ''
or
bobooks.owner like '%'||:P19_OWNER||'%');