I have a suggestion. I noticed, many non-native English speakers try to translate their sentences into English, but they not always perfect in it. It adds some wrong sentences to the English corpus, but, in the other hand, those English translations can help to learn another languages.
So, I suggest to create "Simple English" or "Internation English" corpus, where anyone, who knows English about Intermediate and better, can contribute. These sentences wouldn't mix with the sentences from the main English corpus and wouldn't deteriorate it. Then, English corpus maintainers could transfer good and natural sounds sentences into the main English corpus just by changing the flag, if they like.
It would be possible to create similar auxiliary corpuses not only for English, but for other languages, if there a number of non-native speakers, who wants to contribute and be cheked and corrected.
I read about Slovio, it seems, it requires some training to understand it well.
I don't think, Interlingva is good to understand for English-speaking people, cause English is not very close to any Romance language. Though, I can understand Interlingva a bit, cause I know some Italian. :)