I have a hard time imagining how you can get into business with people in Huaqiangbei without speaking Mandarin even with that guide in hand but without an interpreter. But maybe it can work out with pointing and writing down arabic numbers?
But for millennia, people have gone to far away lands where they don't speak the language, and somehow managed to build trade routes without even having a dictionary or calculator. It is not that hard to work out a pidgin. Tons of things you can do with pointing and gesturing. Marco Polo would have killed to even have Google Translate circa 2010.
I'll also assert with no evidence that it is generally harder for an English speaking engineer to successfully communicate a technical idea into business speak for English speaking VC investors than it is for an English speaking engineer to communicate a request to buy a specific part to a Mandarin speaking engineer.
sounds like a fun challange, probably true
It is surprising how much you can communicate by drawing circuit diagrams, sketches of oscilloscope traces, and equations! It works quite well!
However one problem I encountered was that many (maybe most) vendors were not engineers and had no understanding of the parts sold. They just knew the name of the parts they had in stock, and how much they could sell them for, and that's it. Often they weren't even being paid -- it's a family business and they were just the niece or nephew that got roped in to vaguely watching the store while playing games on their phone.
This was much more of an issue in Vietnam (Nhat Tao market) than in Tokyo (Tokyo Radio tower). In the latter there appeared to be quite a few retired engineers who were quite enthusiastic to meet someone who was looking for something specific. It was pretty neat, and I occasionally encountered someone with a wealth of knowledge!
I somewhat did the same when traveling to foreign countries and meeting people that don't speak any of the languages I speak.
Do learn a little Mandarin, start with Nihao = hello, Xièxiè (shay shay) == thank you - na = that, zhe = this - bu = no, dui = correct - also yuan/kuai the currency (kuai is used interchangeably, a bit like "bucks").
It's all pretty easy, everyone wants to do a deal, they want you to come back as a repeat customer
I'd add: be nice.
A surprising number of the vendors had at least a little english - enough for commerce anyway.
By speaking the same language as they do, which is Cantonese?
Somewhat related, as a result of this, Shenzhen is a great place to try out many different regional Chinese foods.