Victor don’t worry your English is good and you have good rendering skills. You have learned to have good technical skills, but now you must learn to be more creative.
You are probably not getting a reply from UA because they are very busy and because your designs are very similar to other designs already on the market, so they offer nothing new and interesting to them. You must understand that Under Armour has a large team of experienced designers who offer new designs and solutions every day of the year, if you want to compete with them you have to design something unique and distinct that is rare.
You don’t need to show development sketches (unless you are interviewed), but you always need to show a good story for your design. I suggest you think more about a unique theme for your designs. Under Armour has been using much inspiration from nature recently maybe your story can be an evolution of that, like bio-mechanical structures in architecture for example. Also you should offer a unique design solutions like an interesting lacing system, or construction methods. Consider if the shoe can be more environmentally friendly, or help resolve some social crises.
You can also develop the concept of family in “You Are Under Armour” further so it is expressed also in your footwear design. How can family and unity be expressed in one design or an entire design collection? Maybe its a Pride design for a massive Hispanic consumer population in the USA or International markets. You can explore cultural themes as graphics or 3D features like weaving a shoe like a Huarache, or other influences to help connect Under Armour with other consumers. Watch the video below.
Creative solutions can be found in many design dimensions and not just colour and blocking. Think of as many solutions in as many dimensions as you can, then you can choose the strongest ones and the the right combination for your final design.
As Mexican designer you should not be worried about borders, there are designers at Nike, or Adidas from all over the world, Vietnam, India and New Zealand, probably also Mexico.
With the internet you don’t need to travel to meet the companies, if a company likes your work they will interview and hire you, pay for your visa, pay for temporary accommodation, airline tickets, rental car, its not very complicated because they will organize and pay for everything to help you, because you have a precious talent. But before all this you must be prepared to work hard to improve, face rejection and never give up if you want to be hired by a successful company. Eventually you will succeed, but it depends on how determined you are.
There are many footwear designers now working at Adidas and Nike who became known on internet design competitions such as the KicksGuide Artist Series.
Work hard in life and you will be lucky.