Where are the limits of Front Office vs. Back Office

[booblebab]

Hello,

Which of the following sections for translating would I need to translate for only the Front Office and everything customers see to be in that language - while the rest (everything I see) would remain in English?

The following translation sections are shown, but I'm not sure which ones correspond to what:

Front office
Back office
Error messages
Fields name
Modules
PDF

For example, are the error messages for buyers, or me? Fields name? Modules? PDF?

Thanks!