You have reached a certain level of English (presumably intermediate to advanced) by informal learning methods. When you continue in the same way, you may still advance–though perhaps slowly–or feel that your level is stagnating.
There are two things you need to do in order to advance:
- Clearly define your goals.
- Precisely identify your current weaknesses.
With regard to goals, you need a more specific statement than "I realize that English is the key to the job market and research field". Do you want to improve your English for a business environment or for an academic environment? You will need to prioritise one over the other, since you will need different sets of terminology and different types of usage for these two enviroments.
Once you have set a priority, you need to identify more specific, smaller goals, e.g. for an academic environment:
- I want to be able to write a journal article without constantly looking up grammar and vocabulary online.
- I want to be able to teach a 60-minute class about topic X in management science.
Then you need to identify our weaknesses. There are several things you can do for this, e.g.
- Carry a notebook (or take digital notes) to track things that cause problems: finding the English word for x, a grammar mistake, words that you don't understand when listening to the radio or watching a video, etc. This note-taking mechanism should be portable so you can use it whenever you need it.
- If you suspect you have weaknesses in grammar, find a grammar test and check what types of mistakes you made. (E.g. How English Works by Michael Swan and Catherine Walter has a diagnostic test that then tells you what parts of the book to focus on.)
- Identify the specific weaknesses that make it difficult for you to write e-mails (also add which types of e-mails!), teach a class, etc.
These goals and weaknesses can help you determine what areas to focus on, regardless whether you continue learning on your own, with a language partner or by taking a course. For example, when you decide to take a course, you can first enquire about what it will cover and check whether it covers topics that are relevant to your goals and weaknesses. If you decide to continue learning on your own, the list of goals and weaknesses will guide your choice of learning activities and materials. If you decide to learn with a language partner, it will also help you choose activities with your language partner. Of course, you can combine all these learning approaches (formal courses, independent study, a language partner), depending on the time you have.
If you want to objectively measure your level of English, then take an official test. There is a lot of choice, for example, those offered by TELC, IELTS, TOEFL, Cambridge English Language Assessment, etc. These tests will help you determine your level, but the reports won't help you determine your goals, nor are they specific enough to provide information on your weaknesses (you'll have to remember how well you did in each part of the test, which can subjective).
You may also be able to find some informal tests online; these may give you a rough indication of where you are and help you decide which level to choose in an official test.
So I would recommend the following order of steps:
- Find a free or cheap online test that gives you a rough estimate of your level.
- Determine your specific goals and weaknesses.
- Direct your independent learning towards these goals and weaknesses.
- If you find too many weaknesses, especially in general English language skills, take a general course. Ask the language school if they provide an entrance tests to determine your level. This test may be more accurate than the free tests you find online.
- If you find that your weaknesses are mostly in the specific skills you need for an academic or a business environment, than find a course that focuses on that type of English (academic or business).
- If you still want or need an official certificate that identifies your level, you can take one of the official tests listed above. (I don't think any of these test costs less than €100; only you can decide whether the cost is worth the certificate.)