Translating QT Linguist TS-files
The following command assumes you want to translate the contents of the <translation> part of es.ts from Spanish to Catalan, and produces a file ca.ts with translations from the <source> language (typically English) to Catalan:
paste es.ts <(apertium -u -f html es-ca < es.ts) | sed '/<translation/s,^.*\t,,' |sed 's/\t.*//' > ca.ts
It also assumes all <translation> elements are one-line.