Sometimes it can be hard to know if the artist is comfortable with reposting when you don’t speak Japanese. That’s ok! However here are some common Japanese sentences used to prohibit the redistributing of work.
Keep in mind, we don’t speak much Japanese either… so I’m afraid you won’t be able to ask us for help with anything in that language. These are just things that we’ve noticed from Pixiv profiles.
I have refused all use and reproduction of the artwork.
No unauthorised reproduction or use.
And guys, here it is in 5 languages:
・No reproduction or republication without written permission.
・本站内图文请勿随意转载 / 本站内圖文請勿隨意轉載
・게시물 무단 전재 복사 배포 등을 금지합니다
・Gebrauchen die Bilder ohne Genehmigung verboten.
And this one’s specifically tumblr:
Basically, as Iliata pointed out here 無断転載 translates to ‘unauthorised reproduction’ which is basically saying that you aren’t allowed to redistribute their art unless they say you can.
Another way to check is just to google translate the artist’s profile page to see whether they say anything about reproduction.