I notice some Polish words are similar to Spanish. Why is that?
Might have a latin/french root.
What about English English and Scots English? They have at times been considered seperate languages. Until the two countries became one about 300 years ago, it was considered two seperate languages.
There's also the argument that it is a political thing, declaring Scots a 'dialect' rather than a 'language' somehow diminishes it and puts in under English. It's the old "the difference between a language and a dialect; a language is a dialect with a bigger army".
When I was at Glasgow Uni one lad became the first person in the uni's history to write his degree totally in Scots (apart from the papers he wrote in Gaelic). However, this was only made possible by agreeing to consider Scots as a 'variety of English'.
It's important to consider that up until the printing press each area of England and Scotland used it's own regional dialect. With the printing revolution it became more important to use just one main variety, which ended up being the Oxford dialect.