To get to ICAO level 6 as a non-native speaker (which, as hinted above, is a bit of a long term project when you start from scratch / level 4) I would recommend to
- Study a semester or two in the UK or the US (Spain has plenty of sponsorships for exchange students with both countries; maybe do an MBA or some other postgraduate course that benefits you in later life - or in between aviation jobs)
- When you are ready for them, study advanced grammar books such as
to polish your style
(no advertisment intended, do a google search to find good alternatives)
- Take the Eurocontrol spoof exams. Yes they are focused on ATC, but I found them very useful preparation for the exam as they set the required pace for the auditive skills.
- Collect 10 aviation related pictures and print them. Draw one at random and describe what you see in one minute. Record your description (with a voice-recorder or your mobile phone). Listen to the tape to see where you could improve yourself.
Please bear in mind that, irrespective of their ICAO English language level, no one really understands
- Stansted Approach;
- Eckhard Seebohm; and
- EASA