Great, and most importantly, a useful game. 

English is not my native language, but nevertheless, I am making some progress :)

I played the game a bit and for someone who is here for the first time, I didn't immediately understand the objective or what to do.

It would be cool to write one or two advertising phrases, for example:

"Type faster - earn more",

"Compete with friends, the winner is the one who earns more money, quickly and without errors while typing words",

"Be fast as lightning."

And so on...

My impressions:

1. I noticed that the text I enter is case sensitive. This requires a bit more time to input text since you have to use Shift.I would use this to create difficulty levels.At the beginning of the game, all text contains only lowercase letters. As you progress, suddenly one word appears with a capital letter, unexpectedly indeed :).

2. At the end of the sentence, there is a punctuation mark "period." Missed - the period and the word - not counted, cool. I think many people overlook it. By complicating the text, the game becomes more challenging, as there are also other symbols like "?", "!", "()" and so on.

3. When I type a word, it always appears in green. Why is that? My brain got a bit sad. We are all used to green - true, red - false. I see the text is green, I press enter, oh no, I made a mistake. It would be cool to change the text color dynamically, during input it's black, if the word is typed correctly, it turns green.

If the text doesn't turn green during input and I press enter, then for 0.2 seconds it turns red and disappears (a small animation here wouldn't hurt).

You can also add different sounds for successful input and errors.

4. A "Help" button with a brief text and game rules would be useful.

5. A timer can be added (minutes:seconds), the game starts and the timer is set to 01:00, every second counts down, at the end a long beep, time's up your earnings are $134,562,030, not bad, let's call this mode "Competition". Now two friends can compete, how much they will earn in 1 minute, the loser - of course, buys beer. The regular mode (without a timer) can be called "Training".

6. The music in the game is great, I like it.

7. When I make progress, the coefficient of my earnings increases, there is an animation of increase, cool. I would add symbols "+" or "-" before the coefficient, for example: "+0.10", this makes the UI more informative.

8. Why isn't there localization? This needs to be fixed, I can recommend a good translator for Ukrainian.

9. When designing UI/UX for mobile phones, I use the service www.figma.com to design the layout, just drawing it, before starting to write code. This is very helpful because there is a template to follow step by step, besides, you can always show the prototype to others and get feedback, I recommend it.

10. I like that you can buy upgrades and spend honestly earned money!

I couldn't understand what language the code is written in and whether its source code is available on GitHub? 

Looking forward to updates, but for now, I'll try to improve my score :)

These a grammatical error "soccer" doesn't exit it's called Football

nice game, would love to have a configuration for a qwertz instead of a qwerty keyboard layout ;)