For short documents it would be a bit annoying. For long documents it is a possible good solution. We could make it conditional on the length of the text. If the text has more than x characters, check only the selected text. Otherwise, check the whole text.
The link is https://languagetool.org/api/v2/languages. Just for testing that the server is working. What do you expect here?
I thought this was not much needed, because the text is not checked every time after a suggestion is applied. I will look into how it is done in the Chrome extension.
Now there is a button for closing a rule match ("Ignore"), and another one for closing repeated spelling errors ("Ignore all").