How can I get pathauto to generate language-inspecific rules?
This is currently not possible.

Pathauto always uses the language of the entity, just like the node edit form does.

This is a pretty common scenario and there are feature requests for it, for example: https://www.drupal.org/node/2775275.

There is also a related core issue, about allowing that for manual aliases as well: https://www.drupal.org/node/2689459.

What it needs now is someone to work on those. If you have a budget, you could offer a bounty for someone to work on it. The core issue has a needs review patch, so you could help to test that, that would also help if forward.

I'm considering to implement it in pathauto following the same rules as the core issue suggests, based on whether the path field is translatable or not. That shouldn't be too hard to implement, but it will also need tests.

Disclaimer: I am a (the) pathauto maintainer, you can also contact us (my company) to work on it.