The importance of learning languages

I'm not sure why you are using the Latin expressions "in lato sensu" and "in stricto sensu". We don't normally use Latin expressions like that except in very formal writing or if trying to be pretentious. 

"Lingua franca" is an Italian expression that we use as a loan expression in English. It means "a bridge language that two people use to communicate, when neither of them speaks the native language of the other." Example: a Chinese person and a German person wish to have a conversation, but the Chinese person does not speak German, and the German person does not speak Chinese. However, they both speak English, so they can use English as their "lingua franca". I assume that this is what you meant when you said "common language".

"Attractive" is normally a word we would use in the sense of romantic / sexual attraction, so it's not very appropriate in this context. I would suggest that some suitable alternatives could be "in demand" or "sought after".

The importance of learning languages

In lato sensu Learning languages will help you us (<--Avoid using the generic/impersonal "you" when writing. It's grammatically correct, but it does not sound professional.) to find out about different cultures; moreover, it may help you us to get promotions in your careers because being cultivated will make you us attractive to a lot of in the eyes of many institutions . In a stricto sense, the most important language you we should be fluent in is English because it has become the common language in the whole world tries to speak, and all technological systems are designed in this language.

I'll be glad if you correct my mistakes. Thanks in advance.

Here you go, Alaa. Please be aware of your spacing with punctuation: English places the punctuation mark immediately after the word with a space following it.