I decided to buy a German product and not something cheap made in China, because oneyou can use thisese kinds of pliers for many purposes and it would be a pity if they broke too fast.

"one" can be stylistic in English but isn't conversational; I would use "you" instead. "This kind" is technically correct, but at least in American English "these kinds of [things]" is a common phrase that sounds more natural.

As I wrote in one of my earlier contributionjournals, I am installing a heat pump in my house.

Contribution is a bit of a strange word choice here. Contribution implies you have aided a group effort. For example, for a team project at work, you could describe what you have done as a contribution.

I decided to buy a German product and not something cheap made in China, because one can use this kind of pliers for many purposes and it would be a pity if they broke too fastquickly.

It's a bit weird saying it would be a pity here. I would try:

I decided to buy a German product and not something cheap made in China, because one can use this kind of pliers for many purposes and it would be disappointing if they broke quickly.

too fast is correct, quickly is just more standard in this case

To do this job, I had to buy a tool that I was missing: a large pipe wrench.

I think "to do this" sounds better than "to do this job", since saying "job" sounds like an actual job rather than this specific task.

I decided to buy a German product and not something cheap that was made in China, becaussince one can use this kind of pliers for many purposes, and it would be a pity if ithey broke too fastquickly.

Sounds better like this. You should almost never have a comma before because, so I changed it to since.
https://www.grammarly.com/blog/comma-before-because/

I have already tried out my new toy and I find it very pleasant to work with.

sounds better like this. In the earlier part of the sentence, you said "I have tried" which is past tense, and without saying "I" again, it sounds like "I have find it ..." which is wrong. You'd either say "... and found it very pleasant to work with" or add in the "I" and say "I find it very pleasant to work with" to basically reset the tense.

I am sure it will serve me for a very long time.

Technically correct, but I would personally say:
"I am sure it will last for a very long time."
I barely hear people say that something will "serve them"