Generally I use PMCSkin3D for textures https://www.planetminecraft.com/skin-editor/ in 2d mode you can edit other 1:1 or 1:2 aspect ratio textures not just entities. Blockbench is also great since you can see what multiple copies of a texture look like next to each other.
A lot of image editing sites/apps will copy the transparency as well as the color if you use a color picker tool on the texture, then you can see what the opacity value should be.
I mean, I did say "any language vanilla Minecraft supports is fine" so guess I gotta stick to that XD if someone makes a lolcat language file it's going in the mod