They won't make a German AZ for me (even not on English), because it
would highly contradict their company and country standards.
That's pretty obvious I think. They would be unable to do so without hiring a lawyer with knowledge in German labour law and I personally think it would provide no use for you anyway.
The German Arbeitszeugnis is - as you already pointed out yourself - more of a formality. Due to legal restrictions they wouldn't be able to write the truth in there and that's also the reason, why it wouldn't provide any benefit for you having an Arbeitszeugnis from them in my opinion.
Actually I think you overestimate the importance of a Arbeitszeugnis in Germany a bit. Atleast for knowledge workers I wouldn't say it is important at all. Clearly it's important to have one. If your previous job was a fulltime employment at a German company - it would be weird if you had no Arbeitszeugnis from them. But as they are 95% the same (containing the same phrases agreed upon by hundreds of judges and lawyers) it's not actually important what's written inside of it.
What should I suggest to them?
In your position I would just suggest them to write a proper letter of recommendation and make sure that it contains;
- the achievements you made in this position and which benefits resulted for the company,
- the technologies you used,
- some sentences about your work ethics.
Also try to make sure it looks official (written on company letterhead and with personal signature).
If a German boss in the future checks my US LoR, what will be important for him in it?
International project experience is generally well received. As an LoR is pretty obvious not the same as an Arbeitszeugnis, German HR won't be looking for formalities inside of it, but rather see it as a personal reference letter. These are also common in Germany (especially for freelancers) and are free of any formal requirements.
Expanding on the point "Is the Arbeitszeugnis a formality?"
@mart Actually this may of course be a relative and debated opinion and may very well depend on company size/culture and industry sector (the OP is in IT, where I think my claim is especially valid). In my opinion (and I think I'm not alone with that one) the Arbeitszeugnis is worthless for the assesment of an individual applicant based on the following facts:
- As the OP already pointed out in his answer the employer is forced by law to write a generally positive Arbeitszeugnis, which limits his ability to do an actual assessment of the person, which the Arbeitszeugnis should be supposed to deliver.
- Still there are over 30.000 lawsuits over the contents of Arbeitszeugnisse in Germany every year. Law suits in which the employer has nothing to gain, but only to loose (time & money).
- The above points result in the fact that most companies nowadays tend to just hand out a templated Arbeitszeugnis not specific to the actual performance, or an overly positive on. A recent study (from the university Erlangen-Nürnberg) showed that the average grade was 1.9 in 2014 (while it was at 2.4 in 1994). Obviously if the average grade is already that high, there is not much of a possibility to actually compare applicants based on this gradings. If everybody left school with an A-grade, that grades would be pretty useless to find the intellectually gifted ones.
TLDR; The Arbeitszeugnis is on a slow way to meaninglessness since a few decades. Nowaday it's just a formality to have (if your previous employer was a German company - that's why the OP doesn't really need one), but in most of the cases you are likely to gain nothing by having an very good one.
If you don't trust my judgement here, here, here and here is some further reading (in German).