Stats: w810 CID: 49 RED R4EA031 debranded but still locked
I have the custom file AMERIVA_2(CDA102555/107) but didn't that one don't have french language and I want to change to EUROPE_1 CDA102494 eng,fre,pt,es
Q1: can i just flash the phone with the new customize file or i have to flash with MAIN and FS?
Q2: some people use diff FS and Main example FS R4AD001 and Main R4EA031 is there a reason?
Q3: I will have to reinstall all my patches and themes right?

You can flash only the FS, I recommend you use R4ED001, it's a guaranteed match. You can also use EUROPE_3 for french language. Actually, it isn't even necessary to flash the FS, search the forum and look for adding languages in W810. There exists a patch that moves your language files to your 'other' folder, so you can easily edit them. You will lose your themes and such if you flash the FS, so I recommend you to look for the patch.

Edit: kind as I am , I've searched for you: Adding languages

Ok let's say i want to start from scratch so i should reflash my phone with RAED001 FS and R4EA031 Main and EUROPE_1 (i want spanish, french and english) custom file right? and then from there re-add all my patches and sound/cam driver.
Thanks for the link on how to add languages, i've search for like hours couldn't find it i think i need to learn how to use the search function better

Yeah, that should do the trick. However, I had some difficulties adding the dutch language in my phone, even when I used EUROPE_3 that contains the language, but using the tutorial I provided to you, I managed to do so.
Just try if a full flash sorts things out for you, and if that isn't satisfactory, use the tutorial.