...

1) find dspace-api-X.X.jar in [dspace]/webapps/jspui/WEB-INF/lib/
2) unzip it in an external folder
3) modify the Message.properties file
4) copy Message.properties to [dspace]/webapps/jspui/WEB-INF/classes/
5) restart your servlet container 

I'm getting "SolrException: Bad Request" followed by a long query or a "tooManyClauses" Exception

From the related email thread:

The issue is related to the Access Rights Awareness feature introduced in Discovery and affect both the search and the browse system (when SolrBrowseDAO are used, optional in 3.x, by default in 4.0).
The issue here is that a single user is included in several groups - that means that the Solr query will include a filter to limit the results to only items that are visible by group1, group2, .... groupN.
If the number of group is high, you can reach the HTTP header limit (4kB or 8kB by default, see maxHttpHeaderSize Tomcat 7 docs) or a tooManyClauses Exception in Solr, that can be "solved" incrementing this parameter.
https://github.com/DSpace/DSpace/blob/dspace-4_x/dspace/solr/search/conf/solrconfig.xml#L474
When the number is to large you could also consider to disable the Access Rights Awareness feature, commenting this line
https://github.com/DSpace/DSpace/blob/dspace-4_x/dspace/config/spring/api/discovery.xml#L25

Solutions:

in case of exceeding header size:

a) increase maxHttpHeaderSize in Tomcat

in case of tooManyClauses Exception:

b) increment maxBooleanClauses; or

c) disable Access Rights Awareness