Unable to type letters with accents in any game that runs with Proton

#Games affected:
Any game that runs with Proton and requires typing. For example, The Elder Scrolls Online (306130) and Super Animal Royale (843380). The problem doesn't happen in native games, like CS:GO. So, it's a Proton/Wine issue.

#Computer configuration:
Processor: Core i5 4670K @ 3.8GHz.
Motherboard: Gigabyte GA-Z87-D3HP-CF @ BIOS F6.
Memory: 16GB (4 x 4GB) Corsair Vengeance @ 1600MHz.
VGA: EVGA GeForce GTX 1070Ti 8GB SC Black Edition.
OS: Ubuntu 19.10 Eoan Ermine (x64).
VGA Driver: NVidia 440.59.
Proton Version: 5.0-6.

#Problem description:
I'm able to use accents normally in everything, including Steam chat: á, é, í, ó, ú, ã, ñ.

When I open any game that runs with Proton, accents stop working instantly in Steam chat (I can only type the letter, without accent, like a, e, n) and appear like this in any in-game chat (The Elder Scrolls Online and Super Animal Royale, for example): ´a, ´e, ~a. So, all the keys are working, but the language is not being correctly recognized.

The result: when I'm playing, I can't chat with friends in Portuguese or Spanish without absurd grammatical errors. I recently stopped using Windows 10, thanks to the wonderful work you guys have done on Proton. My games run perfectly and this is the only problem, but it's annoying. I tried a lot of solution tips found across the Internet and nothing worked. Seems like this issue has been around for a while.

My system language is Brazilian Portuguese. My keyboard has the Brazilian Portuguese ABNT2 standard QWERTY layout and is working perfectly fine.

The report names Proton 5.0-6, Ubuntu 19.10, and affected games but no source files, tests, or entry points. First reproduce accented input in a Proton game and compare it with Steam chat and a native game; done means accents are recognized correctly in affected Proton games.