Debian packages various tools to help on translation process.
?gettext utilities are the most widely used when translating software. Po4a was created to unify the process, which allows to convert back and forth from other formats to .po.
This document offers tools for translators to different languages
Contents
Editors
.po files, also known as translation catalogs can be edited with an editor respecting the rules of the file, there are specialized programs for handling translations
Specific editors
These editors let you translate without the need to have a profound knowledge of .po catalogs. Plus, those specific editors usually store previously translated words that will help on future translations.
https://screenshots.debian.net/package/gtranslator
gtranslator designed for Gnome
https://screenshots.debian.net/package/lokalize
lokalize designed for KDE
Editor plugins
If you are more used to a editor, there are helpers for some of them.
Other tools can be found in ?L10N, to automate some of the translation and do some spellchecking.