This is one of the most common questions I get asked.
The answer: it depends.
To begin with, if you are able to read this blog you can give a presentation in English! It means that your vocabulary is large enough to communicate with people on detailed subjects. Under the Common European Framework of Reference for Languages (CEFR) this would be around B1-B2. For those of you not familiar with the CEFR levels, you can read more about it here and use these descriptors to find out your English level.
Once you are at that level you can begin to give presentations in English, but depending on the situation you may need a higher level of English.
For example, if you are presenting on a technical subject you will need to have the proper vocabulary for that field to give the presentation effectively. If you aren’t already familiar with the subject area, don’t panic! Spending a some time researching key words and doing some reading on the topic in English should help, especially if you are familiar with the subject in your first language.
You will also need to have a higher level of English if there is some element of audience interaction, such as a question and answer session. When the audience participates or asks questions it means that you can’t prepare everything in advance. You have to be able to improvise on the spot, which is more difficult. To help you prepare for this, try to think of potential questions the audience may ask and prepare some answers. There will still be some unexpected questions, but at least you will be prepared for most of them.
Lastly and most importantly, your English certainly does not have to be perfect. Remember that the audience wants to hear what you have to say. They will forgive you for a few grammar mistakes if your message is important and you speak passionately.