In Magento you can overwrite code in app/code/local or app/code/comunity. The same concept applies to translations. There are 4 levels (or maybe even more) of translation hierarchy. Let jump right into example:
We have language de_DE. First level (priority) is in app/local/de_DE. Here you should have files that start with "Mage_" and ends with ".csv" (every translate file in Magento is csv so it ends with .csv). You might also have translation files like CustomModule_ModuleName.csv where CustomModule is usually company name that created this module.
I will skip 2nd and 3rd level because they are not so important imo (google them 🙂 ). 4th level has the highest priority. If Magento finds translation on this level it will overwrite ANY other translation that already exists. This translations are located in app/design/frontend/default/THEME_NAME/locale/de_DE/translate.csv where THEME_NAME is the theme you are using. Which theme are you using can be found in Magento backend (google it).
Here you will have only ONE file that is called translate.csv. All the "custom" translation for this language should be here because they are all in one place (not like in app/locale/de_DE where you have tons of files).
Also do not forget to CLEAR CACHE!!! 🙂