How not losing your already added translations on every theme or plugin update

A simple way, without losing your already added translations for theme or plugin is to create a copy of traslation in wp-content/languages

You should create this folders structure:

wp-content
– languages
– – plugins
– – – plugin-name-de_DE.po
– – – plugin-name-de_DE.mo
– – themes
– – – theme-name-de_DE.po
– – – theme-name-de_DE.mo