Add script for managing .po files

[nuest]

Ideally, if a msgid is missing in one locale, the little could suggest a translation!

• Asked about tools at https://mattermost.publicknowledgeproject.org/pkp/pl/g99jiiwpc3ba5ewwsts9wwjkcc
• https://cloud.google.com/translate/docs/reference/libraries/v2/php

[nuest]

Closed with 169144351c2cabba529ab3c11def598695dac0c7