Important point, maybe you can mention here to check the signature campaign rules if local boards are accepted. I have seen many members making really great contributions to their local boards buth they have never posted much in the English sections. If they are forced to post the same content in English now due to a signature campaign, the post quality will be affected in a negative way.
When it comes to technical things it's very important to use the right words and explain details. Some Bitcoin or crypto related content is difficult enough on it's own but if you have to explain it in a another language that's even more challenging.
And it's also a benefit for the advertised projects / services to get high quality content in a local language compared to medium-quality posts on page 5 upwards of a thread. The visibility of the local high quality content will be much higher.

Very good read, OP and I think if everyone who is involved in signature campaigns would read your topic (and act accordingly) the number of spam posts would go down a lot.