I have even learned a lot of Swedish
When you are going to speak another language, your head is filled with grammatical rules. With Interlingua, you are spared this. Of course, grammar is important in all languages but in this case it is very easy.
One surprise for me was that my knowledge of Interlingua has helped me to understand the composition of the Swedish language.
I would also like to emphasize the close agreement between spelling and pronunciation in Interlingua. When you hear a new word, you rarely stop to wonder how to spell it. Also, computers easily "understand" Interlingua used in machine translations.