To copy the project terms to the the empty translation boxes in a language, there is a feature called Copy Terms to Translations. It can be accessed using the button with the same name, in the side bar of any language page. Copying the terms doesn't alter the existing translations.
To copy the translations from a language (such as the Reference Language) to the empty translation boxes of another language, export the translations in the first language to a localization file on your computer. Then use the Import Translations from File button in the language page of the second language, to fill in the empty translation boxes with the translations in the first.
If you're a contributor and you don't have access to the language page where to export the localization file with the translations, you can ask your project owner or an administrator to do this for you.
Note: As project owner or administrator, you can set in Project Settings to Export Empty Translations From a language in the localization project. This way, the empty translation boxes in all languages will be filled in upon export with the translations in that language.
When exporting strings with plurals to the Android .xml format, it will also fill in the empty plural forms with translations (in the singular form) from the selected language.