English-Japanese machine translation is still very bad. I think a lot of people have worked on it, but the problem is inherently tough. Also, there is nothing like the hundreds of millions of words of corpora of example translations for machine learning that exist for European languages
I think the main problem is that Japanese requires a lot more context to make meaning clear than most languages. A translation of an English sentence can be pretty good just based on the context of the sentence itself. Japanese doesn't even indicate in any way who the subject of a sentence is. Lots of sentences don't even require a verb.
And it doesn't help that Japanese has a quirky preposition system that doesn't line up with other languages.
If it was limited to adjectives or nouns in Japanese it would still be incredibly useful (personally that's all I really wanted out of the extension). As you say, lots of sentences don't even require a verb, a well placed piece of vocabulary is everything (from my experience living here anyway).
I think the main problem is that Japanese requires a lot more context to make meaning clear than most languages. A translation of an English sentence can be pretty good just based on the context of the sentence itself. Japanese doesn't even indicate in any way who the subject of a sentence is. Lots of sentences don't even require a verb.
And it doesn't help that Japanese has a quirky preposition system that doesn't line up with other languages.