The SVN url has been changed by SourceForge. In your repository root, or in an individual module, run the command:
svn switch --relocate https://apertium.svn.sourceforge.net/svnroot/apertium/ https://svn.code.sf.net/p/apertium/svn/

From Apertium

[edit] Considering GSoC application (Maltese-Arabic pair)

First pitfalls
I still remember bugs in my first 2 programs, they were rather strange: no segfaults, nothing you would expect. Only GUI-s were completely messed up. The code looked all right - because it was ok. It took me ages to discover that I had confused right and left.

Apparently I don't change. I've started working on mt-ar, but got stuck. It looked as if generation of generators (autogens) didn't work properly, which was rather unlikely... And again, it took me some time to realize I was cleverly trying to generate Arabic with my Maltese generator.

Luckily after this discovery things started to work.