Hi James,
Yes your experience as a translator and linguist would be very beneficial to you and those that hire your service. I can speak a bit of Spanish myself after living there for 6 years, but certainly not at a standard to transcribe and translate! I see this service required a lot, especially on the freelance platform of People Per Hour where I predominently pick up my transcribing contracts. You would be very valued at being able to translate and transcribe.
To be a professional, firstly you need to be able to touch type. This means being able to type without looking at the keyboard. Although you would be listening to audio, you need to concentrate on what you are hearing, not looking at the keyboard to type!
You also would need a foot pedal, this speeds up the transcribing process by you being able to keep your hands on the keyboard at all times, then controlling the stop and rewind a few second with your feet. Foot pedals new can be expensive, but there are plenty for sale on Ebay. Just make sure it is a USB foot pedal and not the round-end PS2 fitting.
The software I use to transcribe is Express Scribe Professional, but they have a free version here http://www.nch.com.au/scribe/essetup.exe. When I went back to transcribing years ago, the free version didn’t allow foot pedals, hence buying the Professional version!
There are plenty of places to find practice files until you feel confident enough to start providing a valuable and much needed service.
If there is anything else at all you would like to know, please do not hesitate to get in touch. Please let me know how you get on as well, it would be great to know. :)