Welcome to Flyertalk!

Please don't take any of what I'm about to write as authoritative as these are just my own very informal observations, based partly on things I've heard and partly on experience in similar but by no means identical situations.

First of all... we don't know if you're male or female and if your friend is just a friend or someone with whom you are in a romantic relationship. This does make a difference - immigration officers (not just in the UK) seem to be paranoid about people who may want to come and settle in their country. So - if you're male then it is likely that your friend will be subjected to some fairly close scrutiny, and she will need to be prepared for that. One piece of advice I have repeatedly seen posted here is "only answer what you are asked, but always tell the truth".

Documents... she should really bring English translations of anything that's important (such as confirmation of work or study commitments). Better safe than sorry - better to pay for a translation then to be sent home.

As for a letter from you - yes, good idea. We used to write such letters for a friend with an Algerian passport (she now has a different passport and no longer gets viewed with suspicion by UKBA), and from what she said these letters did ease her passage through Immigration. Her situation was different from your friend's, though, in that (a) our friend speaks fluent English and (b) she was coming to stay with us, a married couple.

Intepreters - my knowledge of this is only based on passing comments by a friend who is an accredited interpreter for UKBA, but I believe these interpreters are mostly freelancers who get called in as and when required. So - if they decided that they wanted to talk to your friend in Spanish this would almost definitely mean a delay while they wait for an interpreter to come in.

As for you guiding her through the process - I doubt they'd allow that, but they may well want to interview you so make sure they have your mobile number.