>> This is not about hiring, it is about what you are allowed to call yourself, i.e. what you are allowed to write into your CV on your buisness card, on your Web site or use for self advertisment.
> If my job title is Software Engineer, then this is of course what it written on my business card that my employer prints for me.
If you are not entitled to do use the title, then it is illegal nevertheless. After all, the law applies regardless of whether it is followed.
>> Job adverisments in German almost exclusively use the term "Software-Entwickler"
> This is just not true.
A search for "Softwareingenieur" or "Software-Ingenieur" on gulp.de, the leading IT freelancer platform in Germany currently gives zero results. If you search for "Software engineer" you get results mixed with "Softwareentwickler". So I went through these results which gave me aprox. 10 cases where they were looking explicitely for an "engineer" ("aprox." because I excluted those where "engineer" was one option among others or were they were mentioning the field "engineering" -- but even those were only a few). In contrast to that, when I searched for "Entwickler" or "developer", I got 275 results.[1] So the proportion of "engineer" is aprox. 3.6 % to that of "developer"/"Entwickler". I think it is okay to call that "almost exclusively".
[1] Some of it are a bit general, so you may not want to count everything. A quick random sample seems to indicate that the number is low nevertheless; so I went with 275, because the end result is unlikely to be significantly different.