What do you think will happen if the whole world spoke one language only?

I think we would have much less cultural diversity.

if the whole world spoke one language only, there is easy to communicate everything but it will be dangerous for the mankind because the interaction between different cultures which the secret of our progressive evolution will lost.

Communication could be easier, but that does not guarantee unity and commonality. For instance, Arabs speak the same language, they have almost the same heritage and history but unfortunately there are conflicts, different interests and priorities amongst 22 countries.

Moreover, one language dominant in the world most likely leads to cancelling lot of cultures instead of diversification. The world could become boring and not attractive although the single-language assumption is unpractical at all.

we may not have a second language to study

make it simple we will not think in various patterns, language means how we are thinking, then knowledge will not improve, diversified thinking process will not be the case if single language is  exist. therefore the languages make wisdom to the world.  

It is imagination only, it will never happen and impossible as per the current situation.

There would be less misunderstanding.

I think the communication would be more easier !

Artificial languages are never accepted, and coercion is not effective. The world is moving toward English but will do so at its own speed.

The situation is not very likely to happen, but let's imagine...My opinion is that this one language would evolve naturally to different geographical dialects (the further two areas would be, the less alike those dialects would be), and in the end those dialects would be different enough to be considered different languages. In addition to this, different geographical areas would be under the influence of different events (other peoples/countries, invasions, etc) and this would have an effect on the development of these languages (loanwords, etc) This hypothetical situation is somehow similar to what happened in the Mediterranean after the Roman Empire collapsed. There was just one language at that time (latin), that broke into different dialects that years later became the current latin-derived languages (Spanish, Italian, Portuguese, Romanian, etc).

Well religious/cultural conflicts would be more personal,  people will always manage to find something new to divide them or to be more exact " categories" them. Because I believe humans tend feel safer when they think they belong to something bigger.