Reusing translations from different projects might lead to licensing issues. Since it's hard to determine compatibility between different projects, Launchpad leaves the responsibility with the translator to make sure they are not infringing on copyright terms.
The guide here should help you decide if you can legally reuse a suggestion you are seeing marked with a legal warning icon ().
When can you simply reuse a translation?
In cases where a translation is consisted of only a few words, you can feel free to reuse a translation. A few words are very likely the same words you would come by yourself, and as a special case, single word translations are the ones you can certainly reuse.
However, if you use a lot suggestions from one particular project, you should still check for license compatibility (see below).
Another common case is if the two projects are using exactly the same free software license, when you are free to make use of the translation under those same terms.
In case a project you are sourcing your translation from is also translated as part of Translation Project, and that specific translation is in public domain (check if the translator is listed as having disclaimed his translations on the teams page), you are free to reuse translations as well.
Also, you can sometimes get an approval from a translator directly, when you can reuse all translations from that person.
Checking for license compatibility
Sometimes, you have to check if the licenses for the two projects (one you are sourcing translations from, other is the one you are translating) are compatible. You need to make sure if the licenses for the "source" project allows reuse in "target" project licensed works.
To check this may sometimes be relatively easy: if a "source" license is a lax license, like revised BSD or X license (when you can freely reuse all translations). In other cases, you have to check for the compatibility between licenses.
For the most common case of reusing in a GPL licensed project, you have to check the GPL license compatibility list. In general, you can reuse both LGPL and GPL licensed translations in another GPL licensed project.