I plan to start a new project using Joomla 3.5 and CB 2.0. I need to develop a custom CB plugin and I would like to know if CB Api as been modified or not.

Need to be more specific as to what API you're wanting to use. Foundation and database global for example will still work fine. We do not just delete global functions or API classes, but deprecate them and remove much later (basically same as Joomla, you'll have several releases to get usages up to speed before legacy API is removed).

I need also to get a french CB version and Kyle told me time ago, CB 2.0 will handle language in another way than the current "define" language. Have you already new french language file ? or is necessary to make some conversion ?

Use CBTxt and not defines and you'll be fine. We're getting rid of define usage gradually in favor of CBTxt as defines use PHP constants, which can't be changed so it wouldn't be possible to do overrides for example with defines.

My best advise is to just continue working as normally. CB 2.0 is close to Beta 3. That is not a Stable build for live sites and should not be used for live sites. In addition to that no 1.x plugin will install on 2.x. So I need to get all our plugins up to date after stable releases. Once everything is ready you'll be able to painlessly upgrade from 1.x to 2.x.

We're not doing major API deprecation until CB 3.0 (class/function removals). CB 2.0 will bridge the gap and simply mark the functions via phpdoc as deprecated. This gives ample time for plugin developers to get up to speed with new API without breaking their existing usages. So in theory a plugin could just change its version tag so it installs on CB 2.0 and be done.

All backends should be done via XML now. Previously this was done with admin PHP files, which are no longer needed (but can still be used).

Frontend still functions the same for fields and tabs. All tooltip usages should call "cbTooltip" instead of trying to use overlib, etc.. but again I can't be more specific without knowing what API you intend to use as I surely don't want to describe every single API functionality change.

With all that said you can develop your plugin now. I have no idea what developer you intend to use and their experience level so they may end up using very little CB API in favor of Joomla API if they're not familiar enough with CB. Upgrade from CB 1.9.1 to CB 2.0 is as simple as installing CB like usual. We provide a single Joomla install package now so it's even easier as you just install 1 extension and you're done. We've migration code in CB 2.0 to migrate any deprecated usages (mostly config usage and terms and conditions usage).

You say CB 2.0 must not be used for live site but by "live" you mean probably "in production" ?

Yes, production/live site usage is not recommended when using a Beta.

If yes is not a big deal because I will need few month before finish the site. Meanwhile I suppose you will release at least a RC version which is supposed to be fine even with a production site.

I can't guarantee any kind of release schedule. Even when RC is released none of our plugins will be up to date yet. I will be updating all our active plugins after Stable.

However if you say is not possible to install CB plugin (such Auto-Action, Conditional and more) under 2.0, I have no other choice than start with 1.9.1 and upgrade later.

Correct, the loophole is to start with 1.9.1, install the plugins, then upgrade to CB 2.0. However doing this we can not guarantee any sort of compatibility to 1.x plugins. CB will mark them as incompatible in plugin management to let you know this. Incompatible plugins will still load and work, but they maybe using API that is out of date, removed, etc.. The biggest compatibility issues are jQuery based at this time, because I've upgraded our jQuery to 1.11.0 (current) along with all our jQuery plugins (and quite a few awesome new ones). We do load the jQuery migrate file, but it's not a guarantee of backwards compatibility for some jQuery functionality.