Why follow an intensive training?
Languages consist of many words. In English, for example, there are more than 1 million. Because of this it is necessary to make a selection which words you need to learn to understand, for which purpose (general, business, educational, technical etc.), and then to activate these words in speech and writing.
An average native speaker knows about 35000 words. The amount of words they actually use is about 12000.
So learning how to be fluent in a foreign language does not happen in trainings of 1 or 2 weeks.
Short, intensive trainings are very effective for the following purposes, however: