Germany
Germany has certified translators – each of the 16 Länder has its own arrangements.
England/USA
In England / USA a translator can sign a statement before a notary public – the notary public witnesses the translator’s signature.

Guidelines
A number of sets of guidelines have been issued in Germany. none are binding, but all are helpful
Here are the guidelines issued by the ADÜ NordTypical advice
Preserve layout of original (if it’s a certificate)
Make sure you have an original or a certified copy
Mention and describe stamps and seals, especially raised seals
Mark the beginning and end of the translation by horizontal lines
Certify: the translation is complete and correct (to the best of your knowledge – Bavaria doesn’t let us say that)
In your certification, state if you saw an original, a certified copy or a copy
If the translation is not complete, point out what is missing
Attach a copy of the original especially if it is hard to read (mark on it any omissions)
Translators’ notes can be in square brackets or footnotes – the footnotes (endnotes) should come before your signature

Ask the customer to decipher handwriting on the certificate if you can’t read it
Ask the customer whether the translation is for the UK, the USA or elsewhere

Some ideas that were new to me (from the BDÜ translators’ regular meeting in Nuremberg)
If you do an abridged translation, attach a copy of the original and mark in highlighter the bits you left out
If you translate out of German, get a second stamp with the wording translated into your target language – otherwise, add a translation of the words on the stamp to your certification