The US Census Office estimates that the US will have 138 million Spanish speakers by 2050, making it the biggest Spanish-speaking nation on Earth.

"Spanish, obviously, because we have a lot of Spanish-speaking people in our country.” That's how Hillary Clinton responded to a question by Rachel Maddow of MSNBC, about what language she would learn if she had the time.

It's an easy answer in many ways because the number of Spanish speakers in the US has increased to nearly 41 million. This would make it the fourth largest Spanish-speaking country in the world, after Mexico (121 million), Colombia (48 million), and Spain (46 million). Yet, twenty-two million of these Spanish speakers are bilingual and can speak English “very well” while only 16 million stated that they don't speak English “very well.”

The high number of Spanish speakers in the US causes some to feel that they are living in a bilingual country. That is true only at a basic level. English is by far the dominant language of the country although attempts in Congress to declare it the official language have failed. Indeed, it'd be impossible to succeed without knowing English.

No American university offers programs to become a lawyer, a doctor, an engineer, or other profession in Spanish. It's true that Spanish is extremely visible in business, government, and education. Yet, to be successful in the US one needs to know English very well. Two Latinos, Marco Rubio and Ted Cruz, are running for president. They owe their careers to the English language although Spanish has been helpful, particularly in the case of Rubio.

Spanish in the US is fundamentally an immigrant language not very different from any other. Its survival in the US depends therefore on the continuous flow of immigration from Spanish-speaking countries, particularly form Mexico. And here is the rub. According to a study of the Pew Hispanic Center, immigration from Mexico last year reflects a "net zero" because more immigrants are returning to their country than coming to the US.

The reasons are varied. The fertility rate in Mexico has decreased considerably in the last several decades. In the 1960s the average Mexican woman had seven children but currently the figure has gone down to 2.2. This decrease in population growth obviously reduces the competition for jobs. The Mexican economy has been improving and the appeal of leaving for the green grass across the northern border has become less attractive. In addition, coming to the US has become extremely difficult. The increased control at the border has made it virtually impossible to get into the US without the use of a smuggler, which can cost several thousand dollars.

As immigration from Mexico goes down, the same happens to the vitality of the Spanish language in the US. Spanish speakers increasingly integrate and their forebears' language will eventually become a faint memory as it happens with other immigrant languages.

Yet, the importance of Spanish as a global language will continue. The purchasing power of the Spanish-speaking population already represents 10 percent of the global GDP. The increasing purchasing power of Mexicans who stay home will expand it, adding value to the Spanish language.