1. English.
2. Most US embassy websites are bilingual: English and the given country's native language. Your fiance can use an interpreter at the interview. However, it is advisable that the petitioner attends the interview as well, especially if the beneficiary's English is poor.
3. Most couples obviously live apart during the K1 process, the petitioner is in the US and the beneficiary is in his/her home country. It is up to you how you solve the language problems. Do you even speak the beneficiary's native language? If not, then it could be a red flag to the embassy, they might will be wondering how did you two establish a relationship without even speaking a common language fluently?
4. Yes, everything must be translated to English. Most people use certified translations.
5. Just Google it. It is better to start obtaining those papers as soon as you can, if the beneficiary lived in more countries.
6. Once your case arrives to the US embassy of the beneficiary's country, the embassy will mail an instruction package to the beneficiary. The instructions are very specific and clear and obviously vary from country to country
