xmlsitemap module generates wrong links for languages

Perhaps create an issue in the module's issue queue on github?

While problem is there, you may want to try simplified  solution: Easy XML sitemap.

If that works better for you, then so be it.
But what did you mean when you said, "that is not language sensitive either"?

No one has reported such a problem. 

If you could provide reproducible steps to replicate the problem, it might help us resolve it.

I confirm that in my practice when developing multilingual sites, the XML sitemap internationalization module also causes problems, and with me this is most evident when translating taxonomy dictionaries.

Instead of creating a link in the sitemap with the address of the translated term on the page (URL Alias), the sitemap displays addresses of the type taxonomy/term/11, and this is on the original page, which was then translated into different languages.

I found a workaround, which I suppose may be useful for other people and other cases.

I completely disabled the XML sitemap internationalization module.

In the XML sitemap settings, I turned off the automatic inclusion of pages for all term dictionaries.

I used Views to create an administrative view that displays all terms and allows filtering them by dictionary, by language, by showing the URL of the term.

Then I entered all term addresses using the XML sitemap custom module. Fortunately, creating custom links works flawlessly.

This allowed me to create a sitemap with terms translated into different languages.

In this way, I have a properly working site, despite the problems with XML sitemap internationalization, which I believe will be resolved sooner or later.

Finally, I want to share my understanding that Backdrop CMS and many of the modules that are available have some problems when working with multilingual sites. These problems are gradually being corrected and fixed.

Despite individual difficulties, overall Backdrop CMS is an invaluable and incomparable open source project with exceptional flexibility and power. And the problems can be overcome or the site can be constructed in such a way as to avoid their occurrence.

It's like driving a car that has some problems, for example, the right window won't open. Well, you don't throw the car away because of that, you just don't open the right window until you find a service where they can repair it.

In our case, you don't look for a service, you write to https://github.com/backdrop/backdrop-issues/issues

And if you have a github account - just create one. It's easy.

Many thanks to the Backdrop community for the incredible work they do to preserve the legacy of Drupal 7 and adapt it to modern conditions.

I've written before about the problems with multilingual websites:

https://forum.backdropcms.org/forum/no-way-translate-site-information?page=0%2C0#comment-8313

I'm currently developing a simpler multilingual site with two dozen pages where I only needed to translate individual pages into English, as well as a few blocks and the navigation. I did this without any problems with the built-in tools in the core. I didn't have to install i18n at all.

 

The first problem is not a problem, things work as designed. When you install Backdrop in English, the existing content does not have a selected language.
Then, when you add other languages and make the content type translatable, you need to re-save the existing nodes with the English language selected so that they are translatable.

Second - I repeated your steps.

Sitemap with Turkish:

Then: 

The Turkish paths disappear from the sitemap when I exclude this language and rebuild the sitemap.
 

Hope this help you.

The only help I can offer is related to the module Easy XML sitemap.  

I do not use XML Sitemap because it's too overwhelming (IMHO) for my needs.

I apologize for any potential misunderstanding.

I don't fully understand why someone would need multiple sitemaps for different languages, but you certainly know better. Good luck!