* Lexical similarity: How many words are shared? This is often used to define language families, but doesn't tell the whole story.
* Grammatical similarity: How similar are the rules of syntax and morphology?
* Phonological similarity: How similar are the sounds of the languages?
* Historical relationship: Do they share a common ancestor?
Here are some examples of language pairs that are considered highly similar:
* Serbian and Croatian: These are often considered to be mutually intelligible, meaning speakers of each can understand the other without much difficulty. They are very similar in vocabulary, grammar, and pronunciation, largely due to their shared history and close geographical proximity.
* Danish and Norwegian: These languages are also highly mutually intelligible, especially in their written forms. They share a common ancestor (Old Norse) and have a close relationship.
* Hindi and Urdu: These languages are very similar in vocabulary and grammar, but differ slightly in pronunciation and script. They are largely mutually intelligible, especially in written form.
Ultimately, the answer to which two languages are the *most* similar depends on what aspect of similarity you're looking at.
It's also worth noting that even languages that seem very similar can have subtle differences that make them challenging for speakers to fully understand.