Okay,
Tensei Shitara Slime Datta Ken
The top followed manga has just over 40k followers on this site alone. That means that each chapter raw bought that "supports" the Mangaka potentially has been read by 40k on mangadex let alone the scraper-bot sites (plus guests who don't follow but read/minus those follow but don't read) This means that the 2, 000 yen from buying that one volume/magazine, yes I'm being extremely generous got read by 40k people; again large estimate.
It really doesn't matter if scanlators from each language actually bought that one raw copy or asked from the Eng scanlator for the raw. Sure imagining that 50 scanlators from different languages brought their own copy. That potentially gives the Mangaka 100,000 yen compared to 2,000yen . But it doesn't compare to the potential lost of 40,000 x 2000yen (80,000,000yen) of the Mangaka because we have scanlators translating it and distributing for free.
Restricting languages because they get their raws from Eng scanlators is useless to put it mildly in the grand scheme of things.
About 0.122% difference between 100,000 and 2,000 compared to 80,000,000.
Either shut down all scanlators so that people have to learn jap to read manga or sell official translations. Neither of which are being done by publishers properly as of right now.
Spreading the word via scanlators allows for those who don't understand Japanese to find gems that they will buy merch or physical Japanese copies (that they can't read) for decorations just for the purpose of support the Mangaka.
I don't how else other than using numbers to explain this you. Yes scanlators of any language is detrimental to the sales of the Mangaka. No this does not exclude English(to be honest English and Chinese translations are probs the most detrimental though cause most read in those two languages).
If you find my points lacking. You can probs tag some group leaders or Md staff and ask them why they allow for other languages to scanlate other than English.
Edit: grammar and calculating actual number instead of providing method of calculatuon