Firstly, I think that your concern regarding locale/languages should be resolved. The only other places I can think of having that solved if no-one else here is able to at the moment and who may have some experience in dealing with it is at the Arch forums. The people there are fairly knowledgeable; however, for reasons that are far beyond me to understand, they don’t like n00bs. They seem to be a bit hostile towards them. Unlike here on Antergos forums where the reception is quite different as I"m sure you’ve become aware of. The Arch Wiki is an incredible resource to use. I use it multiple times for many things.
I would at least try going there on the Arch Forums on a way to solve this issue because I think it has merits in finding a resolution for it. I would go ahead and use the outputs that you’ve generated in the above posts you submitted along with what I’ve suggested to try for the [i:3rp6bquh].conf[/i:3rp6bquh] files.
If you’ve used the file from Launchpad, mention that as well. Mention pretty much all that I’ve given you to do to try and post those outputs on a new post there. You will be expected to search their Wiki as well, which I’ve already mentioned, is a fantastic resource.
The other place I can suggest is the Ubuntu forums. The have a section dedicated there for other OS. The place there is filled with very knowledgeable people. The knowledge resource from the Ubuntu forums is unfathomable. The place is very n00b friendly, too.
As for your boot related problems, sorry but that I don’t know of either in how to help get you past that. I find that Antergos not being able to filter and find the icon for LM is kind of strange as it can find the WIndows icon. Try looking in the directory[i:3rp6bquh] /boot/grub/themes/[/i:3rp6bquh].
Keep in mind that the icon might not even exist. You might have to make one up or find one on the net. It would most likely be a [i:3rp6bquh].png[/i:3rp6bquh] icon. If the icon doesn’t exist, take a look at other icons for the other OS using the photo editor that comes with MATE. Determine what the pixel size is for each icon as you’ll have to adapt your LM icon to the same size as the others. Otherwise Grub may or may not accept some other size. Then you’ll have to
grub-mkconfig -o /boot/grub/grub.cfg
There may or may not be other files that you’ll have to mess with to have grub find the icons for LM. There is probably a file somewhere that points grub to the directory to where the icons are for all the OS.