"As for Japanese and Korean, both are still generally regarded as isolates. There have been some vigorous attempts at relating them to each other, and further to the (very controversial) proposed Altaic grouping, but even the linguists pursuing this admit that they have not so far obtained conclusive evidence, or even moderately persuasive evidence, and any claims to the contrary are at best premature.
However, only if the languages are so much alike that they are mutually intelligible, or nearly so, or are clearly such that knowing one, you could gain considerable passive command of the other in a few days. But Korean and Japanese, while probably (some will disagree with this) related, are not that close. But the grammars are very similar and it is often possible to translate from colloquial Japanese into colloquial Korean morpheme by morpheme. We cant even do that with German, a language fairly closely related to English.
What shows languages to be related however, is patters of regular differences of sounds in words of similar meaning."
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