This searching mode is based on matching the same original string (msgid) in an auxiliary PO file, translated in other language. It's very common that romanic languages have similar words. Same for anglosaxone and slavone ones. For example, I want to translate then word "on" in Romanian from kdelibs.po, but have no clue. I look in the same file for French and found "actif". In Spanish one I found "activado". So, I conclude that the best one in Romanian will be "activ". KBabel automate this task. Currently you can define only one auxiliary file to search.
You can configure this searching mode by selecting Settings->Configure Dictionary->PO Auxiliary in KBabel menu. In "Configure Dictionary PO Auxiliary" dialog you can select the path to auxiliary PO file. To automate PO file switching when you change current edited file, there are many variables delimited by "@" char which are replaced by appropriate values:
The name of currently translated application or package. For example can expand to: kbabel, kdelibs, konqueror etc.
The language code. For example can expand to: de, ro, fr etc.
where "n" is a positive integer. This expand to the "n"-th directory counted from filename (right to left).