- Can I use Flare and its art to create a commercial game?

The art and code portions will continue to be free/libre so anyone can share them. If you distribute your fully-original campaign data as a stand-alone mod, just that data can probably remain proprietary. You can sell all of it commercially (though people are free to share the core art/code portions, and any specific derivative works).

- Does/Will it support localization, dialogs, effects like poison, fire damage or area effects.

It supports localization, dialogs, and some kinds of spell effects. Version 0.18 will expand upon the variety of effects.

- Will/can I give custom attributes to the characters?

The four main attributes (Physical, Magical, Offense, Defense) are somewhat set in stone. You can rename them and tweak the size of the bonuses they give. For completely changing the attribute system (let's say, changing to D&D style or GURPS style attributes) you'd need to dig into the code and create a fork.

Maybe in the distant future we'll make these customizable, but it would probably require converting the entire engine to use a scripting language.

- Do you plan to provide any tool sets like graphical editors etc?

The Tiled Map Editor is used to make maps. Besides that, everything else is editing plain text files (or creating art). I don't plan on providing graphical editors.

- Will the games run under Windows, Mac and Linux? Perhaps on mobile devices, too?

Windows, Mac, Linux. It can be ported to mobile devices that have SDL support. Note that the GPL may be incompatible with some app store terms.

- One more thing: I noticed that there is quite a long loading time when moving into another area. Will this speed up in the next versions of Flare?

Load time varies by system (on the largest maps, my laptop loads in about 3 seconds; my desktop loads in about 0.25 seconds). There are some optimizations we can do that should help in the future.

Clint's the project maintainer, but I can try for a first pass.  If you want to see just how flexible the engine is, play both FLARE and Polymorphable - very different games, with 99.9% shared code base.

-It very much supports localization - there's a ton of language translations for the game already.  Download the game and you can check out dialogs, as well as most of the effects that are already incorporated.  I don't believe there's poison, but maybe bleed is similar enough.  Fire and ice damage are implemented, and there is a goal to get damage fully configurable.  Plenty of AoEs around.

- Yes!  We've customized almost all of the character stats in Polymorphable.

- I don't think there are any plans for graphical editors, but most of us use GIMP to great effect, and Tiled is the mapmaking tool we use.

As far as loading time, it's been pretty good to me.  

FLARE is still in Alpha stage as well, though it is pretty stable.  If you're eager to see it progress, I would say, fork the project and see if there's anything you want to contribute to it!  That could be artwork, code, or just playtesting!

EDIT:  He totally beat me to it.