For example, I am French, I usually provide French translations with my plugins that actually are fr_FR, but someone with a fr_CA install won't get them despite the fact that fr_FR will likely be good enough.

so the idea would be to let me provide fr only files that would be loaded in a fr_CA settings instead of falling back to English.

I actually wrote a blog post about that, with code, here: http://vedovini.net/2013/12/smart-fallback-mechanism-for-loading-text-domains-in-wordpress/

I'm working with French WordPress core.
It's annoying when I try to install a plugin but the system always force to use the french translation for the plugin.

It's better if we have an option to use the translation or not when we install themes or plugins.

https://developer.wordpress.org/themes/functionality/localization/#translate-po-file

and I recommend adding LingoHub (https://lingohub.com/) as another tool, because there you can easily upload your .po file and translate it in other languages!

You can even sync your files with a Github/Bitbucket repository.

Having multiple languages on a blog is a need for a lot of bloggers (especially in those countries where there are more official languages), I know that there are several plugins around, but...

What if the plugin developer decides to quit, and the plugin becomes incompatible with the newer versions of WP?

The fear shared among many multilingual bloggers is that the they can risk losing the content every time a new version of WP is released.

Every solution has some kind of flaw into it, that makes upgrading WP more complicated than it is.
They either manipulate the DB, or use templates.

The solution in the Codex, http://codex.wordpress.org/Multilingual_WordPress , hinting at the use of multiple blogs could be the best one, however there is no known plugin that actually ties two posts from two different blogs as translation of each other.

I did my research, and found the following threads, however they are currently closed, (otherwise I would have posted in them):

http://wordpress.org/extend/ideas/topic/support-multiple-languages

http://wordpress.org/extend/ideas/topic/make-wordpress-multilingual-friendly

http://wordpress.org/extend/ideas/topic/multilingual

http://wordpress.org/extend/ideas/topic/multilingual-blogs-posting

http://wordpress.org/extend/ideas/topic/true-multilingual-platform

Many of those suggestions have been deemed has "plugin territory", but for the reasons stated before, a plugin not mantained by the core team can bring several problems with it, and scare bloggers away.

Should a multilingual support see the light as a core enhancement?

I take this opportunity to briefly esprimiere another idea / complaint (I do not know where to write it). When you go to the head with Wordpress text it is not formatted properly! Why? Let me give an example. This is the text that I write:

"Bla bla bla il ranocchio le disse: vuoi prendermi in sposo? La ragazza rispose

che non voleva sposare un rospo e così il matrimonio non si celebrò bla bla bla..."

Note that I went to the head twice (I left a blank space between the first and second line). But then if the public the text formatted risuterà "all attacked" like this:

"Bla bla il rospo le disse: vuoi sposarmi? La fanciulla rispose che non desiderava sposarsi con un ranocchio e così il matrimonio non si fece bla bla..."

Even if I press Shift + Enter (as some guides recommend) the problem remains. Several guides online intending to use the code to overcome this problem. You can fix?

<name>
   <en>Sample plugin</en>
   <fr>Sample plugin in french</fr>
</name>

This is especially important in multisite environments where users have the option to enable plugins or activate themes, but they can't read the details.
The theme and plugin repositories should be multilingual too! :)

Joomla included it since version 1.7.

I am asking you to include built in multilingual plugin in wordpress.