Word order and the words themselves

I was asked the following interesting question the other day, which I will try to answer here as I think that it will be of general interest.
“Have you had any other language experiments? Or is it just Spanish? If you had what was the difference?”
The first language that I’d say I really mastered was Spanish, but after this I caught the bug for learning languages and also having learnt one, this meant I then had an idea of how to go about learning other languages.  The next language I learnt was French, this was made easier because the word order is similar to Spanish, and also because I remembered some of the more basic words from school.
After that I moved back to England but near the Welsh border and started learning Welsh.  This is a completely different language to Spanish, French, or English, so in learning this I had to learn a very different way of thinking.  Also the vocabulary is very different so this meant I had to learn the new words in a different way, with Spanish there are many words that are similar to words in English, similarly with French but with the added benefit of words similar to Spanish words.
I’ve since experimented with various other languages all to various levels of knowledge or fluency, these include:
German, Dutch, Portuguese, Italian, Catalan and Japanese.
The differences in all these experiments are most apparent in two aspects (the word order and the words themselves) and how these two aspects relate to the language(s) that you already know.
For example, if you just know English and want to learn Spanish, then looking at these two aspects, word order and the words themselves, you’ll have the following things to consider:
1. Word order
Spanish has a different word order a lot of the time, this means that you’ll have to change the way your brain forms thoughts when speaking.
2. The words themselves
In Spanish there’s quite a large crossover of certain types of words, particularly with academic or literary vocabulary.  However a lot of the basic vocabulary is very different, this means that you’ll have to learn a lot of very different words at the very start, but then later on it will be easier to pick up more advanced words.
With each language you have to work out how to get progress in these two aspects.  
There are then additional aspects, for example particular grammar points (which I would group close to the word order), irregular pronunciation (for example with French), new alphabets (Arabic, Russian, etc) or other features.  However the two most important aspects are the two I’ve mentioned above.