WhiteBear

CERTIFY means the translator promises, vows, assures that he/she is fluent in both languages and the translation is accurate. It could also say "I promise I am fluent.......etc". The translator does not have to be a paid professional service. If you want to pay a fee, then that paid person will also certify (or promise) they are fluent and did it correctly. They actually don't. All embassies have slightly different procedures.London says:
Translations: All documents not in English must be accompanied by certified English translations. The translation must include a statement signed by the translator that state that the translation is accurate. Only one copy is needed. Note: Applicants are also required to submit the original document, in the original language.
If they wanted a professional paid translation service, they would say so. If they required a notary to witness the translator signing, they would state that was required.
I am wondering where the interview will be ....London or Latvia. I ask because I saw you started a thread asking where to get a visa medical in Latvia.
Fill out your timeline with visa type and embassy.

White Bear--FILL OUT YOUR TIMELINE. List the visa type.
Read this thread. The first post discusses immunizations and answers your question about the statement. http://www.visajourney.com/forums/topic/474161-london-k1-a-complete-guide/
You have a choice:
1) Get the shots free on National Health before going to the medical exam and be signed off by the doctor at Knightsbridge as done with immunizations.
2). Wait until you get to the US. Pay over $100 for the shots. Call 10 civil surgeons trying to find one who won't make you pay to take the whole medical exam again. Spend an additional $100+ to pay the doctor to fill out a new immunization form because you didn't get it done when you had the chance in the UK. If you get real lucky, maybe he will only charge $50 for filling out your form. If you are not knowledgeable (which you aren't) a doctor could charge you $250 for his fee plus additional for a TB test and immunizations.

Completely agree with this. Always get an extra copy of whatever document you require as a "just in case".

It's because you have to pay them before they process your application, and the amount you pay is dependent on what you request.
So for example if I ordered the standard service at £45 plus three additional copies at £6 each, that would be £45 + £18. So my total would be £63. I'd have responsibility for making sure my postal order was for £63.
If I didn't add it up properly and put, let's say, £65 on the order, they would just donate the extra £2 to charity. I guess it doesn't happen that often, but there are people that are unfortunately silly enough to not be able to add up their total amount properly. Similarly, if you underpay, say if I put £62 on my order, they'd call you to request the additional money needed to process your certificate.