You guys use MSYS2 to compile your custom ScummVMs? /j
Anyway, with regards to how type a given character: on today's modern fully Unicode OSes, or at the very least modern Windows, the keyboard layout merely determines which characters you can type, and how. Pressing the "E" key doesn't tell the application to process "E", it tells the system you pressed a key with scancode 0x12. The system then determines from the layout you specified that scancode 0x12 means "E" and sends that to the focused application. Likewise, on a French layout, scancode 0x03 with no modifiers means e-acute, but for me an unmodified scancode 0x03 means "2". And on a Hungarian keyboard, the dedicated e-acute key is next to the "L" with scancode 0x27. For me, that's a semicolon.
But no matter if you use deadkeys, altgr, or a physical dedicated e-acute key, you're getting character U+00E9 and that's how it's understood.
However! I don't know how ScummVM's language settings are supposed to influence keyboard layouts. And even in the year of our Luigi 2025, there are applications that are not made Unicode-aware. Such as SCI Companion, which is violently non-Unicode! If you run it on your average American or Western European Latin alphabet using computer, it'll use the Win-1252 code page. If your system is set up to use Greek for non-Unicode applications (or you used AppLocale or its ilk to switch locales for just that application) it'll implicitly use Win-1253 instead, replacing the upper 128 characters and changing the meaning of the byte 0xE9 from e-acute (U+00E9) to lowercase kappa (U+03BA).
And if you set it to Japanese and disable the DOS to Windows remap feature in SCI Companion, it can correctly show and edit the text for PC-98 Police Quest II. Isn't that wild.