The translation scheme was never great but for a long time it was good enough, now when a bigger and bigger part of typical UI of a site is plugin based, it is just a meh - just better then nothing.

The problem with translations is that some times to provide good translation you actually need to see the string in its context which means that it is not enough to have a good control of english but you also need to be able to read code in order to understand how to make the text appear. Proper translators will not know how to do it, and for coders it is too boring. Translating wordpress core is not always easy and there is no money or glory in it.

What should be done is to have a micro translation utility in core to help site owners translate or just modify translations. This will help to make the translations which are 90% ready useful, and will help when the translation is just out of context in a specific site and with plugins. For example even on english sites you might want to replace the tecno gibrish of "posts" with "articles" or "news items". There is actually a plugin that does it, but I forgot the name of it :(