Is it true that once one has mastered a language other than one's mother tongue, that it will be easier to learn a third or fourth language?

I don't think you necessarily need to master it but certainly learning one language can help you to learn others. I think it has a lot to do with perspective on how grammer isn't the same, and translations are often imperfect.

Well, the skills involved in learning a language are pretty versatile, once you learn one language, you will be familiar with the process of learning a language, so that should make learnig any langauge a bit easier.

Yes, it makes it somewhat easier because you are familiar with language learning. If you are learning another of the same type of foreign language, then you will also see the associations between the two, though if they are too alike it can be confusing sometimes. But either way, you already know about the parts of speech, different sentence structures, etc. Although learning a 3rd language can mean learning different of all of these, at least you are familiar with the whole learning process. Good luck.

Depends on language. Most languages are in essence from four mother languages. Hebrew, Arabic, Greek and Spanish. Leaving out the Chinese and Japanese languages these languages are what connects us to particular cultures and peoples.

Learning down from these languages makes it easier to learn a similar language. So a Dutch speaker will have an easier time learning Flemish an Arabic speaker Urdu and Hebrew speaker Russian etc.

English is funny in that it isn't really a language at all but a mixture of languages which is why next to Chinese it is the most spoken language in the work as most of the words are used natively in the local languages.

There can be funny translations though or misunderstandings. My favourite was visiting India with my then girlfriend Laura, a name which literally translated into the local language means fat penis….

Yes because you are no longer held back by fears such as embarrassing yourself by speaking in another language and making mistakes. Also, as many languages are related to others you can learn similar ones more easily.

i dont think so.
i mean i no spanish. and that was very hard to learn. and im still struggling to learn it now.
And now im into French and boy let me tell you.
thats just as hard as the spanish im trying to learn.