Even if one could verify grammatical correctness, there are many ways to produce unnatural Japanese phrases. To give an easy example: 9つ (9 things) is natural,

Even if one could verify grammatical correctness, there are many ways to produce unnatural Japanese phrases. To give an easy example: 9つ (9 things) is natural, but 10つ sounds extremely strange. However, 10個 sounds fine. When the number is large enough, it's also common to not use 助数詞 at all. Sometimes, grammatical mistakes are natural Japanese. For instance, there is a concept of ら抜き言葉 (words with ら dropped), where people will say e.g. 寝れない ("I can't sleep") instead of 寝られない. This is an error in conjugation, yet it's natural language and applies to a few other words, too. Validating both grammar and word choice is still insufficient to judge naturality of a Japanese phrase. A common "mistake" made by many Japanese is writing 「違和感を感じる」. The verb is redundant because of the 「感」 in 「違和感」. The "correct" word to use in this case is 覚える. In practice, however, either choice of word is understandable and considered correct (except to those with the trivia of 「違和感は覚えるもの！」) Sometimes, redundancy makes phrases considered incorrect (see 二重敬語 for an example). In other cases, nobody will debate the correctness of the phrase.