Mostly decent, but British English is not listed as a mother tongue or preferred language, so I was constantly get 'corrections' from British English to American.
I CANNOT have English 'no variant' as ONLY option shows US flag for both mother tongue and preferred language. The only place where English British is available is variants which doesn't seem to ever come into play when I was using this as it only ever flagged up British spellings wanting to change them to American.
I'd post screenshots but i cant seem to be able to.

I downloaded it because it advertised itself as working for multiple languages. While it did appear to work fine for English and French, it unfortunately failed to catch basic Japanese grammar mistakes such as missing particles and misspelled words. Given that I need this most for Japanese (as that is my weakest language at the moment), I have decided to remove it from my browser.

The spell check system automatically checks punctuation first and highlights text in yellow ...which automatically blocks/disables spell check itself, how come there is no way to configure the order of these options

Mediocre results.
*New words you add to dictionary are never offered as correction.
*Many times there's no proper correction for simple typing mistake, like changing the order of two letters in a word - the suggestions are weird and irrelevant.
TL;DR - barely more useful than Firefox' (horrible) built-in speller.

It's a very useful tool for spell checking but I'm very sad that it's not available in Hungarian. I know it's a difficult task to include all languages but I still expected all EU countries to be available. Disappointed.
Still, the app works as advertised when I'm using English.

В целом хорошо. Но попадаются обычные слова которые не знает, отсутствуют в словаре. Большим недостатком является отсутствие проверки на букву ё. При этом блокирует встроенную проверку от браузера, где есть проверка на ё. Поэтому вынужден отказаться пока от данного расширения.

It's okay, but I have a bug where it doesn't check commas and punctuation.

And also on some form fields (like this review) it just doesn't work at all.

Until recently, it was really great, but since the last update, it's become quite annoying. Every time there's a suggestion for correction, two windows pop up simultaneously, which you always have to close with a mouse click, and that's killing my nervs. If you, like me, use a lot of specialized terminology that the language tool doesn't recognize yet, the entire text is constantly plastered with suggestions. It makes working impossible. Additionally, it now only wants to correct into Dutch, even though German and English are selected. This renders the tool completely unusable.

Mostly works well, but lately it puts two dialogue boxes, one of which does not work.

Wenn es nicht oft die falsche Sprache erkennen und dann alles als falsch markieren und sinnfreie Vorschläge machen würde oder man die automatische Erkennung ausschalten könnte, wäre es richtig gut. Ich habe Muttersprache Deutsch und bevorzugte Sprache Deutsch und dann kommt es mit Niederländisch daher und macht alle Vorschläge in Niederländisch.

Nachtrag zur Antwort.
Danke, das habe ich schon mal gemacht, brachte aber keine Lösung. Das Problem tritt vorwiegend bei Google Notizen auf und selbst wenn man händisch auf Deutsch stellt, ist es nächstes mal wieder Niederländisch. Am besten wäre wohl, die automatische Erkennung auszuschalten.

Just started using it. We shall see.

Excellent product. However, not satisfied with the add-on. The inline pop-up closes every time the page is scrolled. My display is 1280x, and I use a mouse, and i will have to scroll to see the language suggestions. It becomes very annoying to reopen often. Need an option to close manually only using the close button, it should not close on page scroll. It also sometimes closes itself, when fetching paraphrasing suggestions, must be a bug.
Update: Posted, and pending.

After 2 years, it mostly cannot detect and disable checking when writing in unsupported language (Czech).

I would rate 5 stars but I think you should be able to turn of the notification that the text has advanced errors. I understand that they need a subscription of that but I think we should have the option to not accept, I would rather it just ask me if I wanted to upgrade every once in a while rather than always tell me that there is a problem and not telling me how to fix it.