Everyone at Tatoeba should be familiar with the wiki page Contributing in a language that is not your strongest. In particular, note the first point: "Only write sentences in a language you know very well." If you contribute sentences in a language that you do not know very well, your errors will overwhelm the ability of others to correct them.
If you see a bad sentence, you should leave a comment on it. Ideally, your comment should indicate how to correct the sentence. If you are able to leave tags, also add the "@change" tag.
If you see someone contributing a large number of sentences with errors, also do the following:
(1) If you are a corpus maintainer (or admin), send the contributor a private message asking them to stop contributing in that language, at least temporarily, and fix the errors that have been reported. If you are not a corpus maintainer, send a private note to a corpus maintainer or admin reporting the issue.
(2) If the contributor fails to fix the errors and/or continues to make an unacceptable number of errors, the corpus maintainer should ask the contributor to stop contributing in that language.
(3) If the contributor fails to stop contributing sentences with a large number of errors, the corpus maintainer should contact an admin and request that the contributor's ability to contribute in any language be suspended.