I cannot reproduce this problem here.
Please attach always the DXF/DWG file you are working with. This will tell us what font you are using, what type of text, etc. thanks.

The file will not work because the existing files open, show the correct characters, but I can not type new characters in these files or new files.
Sorry.

I don't understand, sorry.
Do you mean that the text dialog does not show these characters when you press the appropriate keys? Or the text entity created in the drawing area does not show these characters?
In any case: please submit the DXF file (even if these characters are not visible in it).

I tested it on two different machines with Linux Mint 18.1 and it is not possible to type Latin characters composed of two keys (symbol + letter) such as "a, ê, õ, ô, etc." but "ç" that is typed By a single key is accepted. This occurs both in the text box and in the Block Attributes box, when the text or attribute is inserted in the drawing appears without the Latin characters; I also noticed that the DXF file created by version 3.15.5 that contains the correct Latin characters sometimes causes QCad 3.16.5 to abort when opened; Follow attachments.

Note: Disregard the file Qcad 3.15.6.desktop

Thanks

Can you please extract the attached plugins into your QCAD directory (32bit or 64bit, depending on your platform), so you get a folder "platforminputcontexts" in the QCAD installation directory, same level as "platforms":

qcad-3.16.5-pro-linux-x86_64/platforminputcontexts/libibusplatforminputcontextplugin.so
qcad-3.16.5-pro-linux-x86_64/platforminputcontexts/libcomposeplatforminputcontextplugin.so

This might fix the problem, at least partially. Unfortunately, there are some known issues with Qt, the framework QCAD uses and keyboard input under Linux:

https://bugreports.qt.io/browse/QTBUG-49438
https://bugreports.qt.io/browse/QTBUG-56452

Problem solved, thank you.

To reproduce:
- install Keyboard layout "German (Switzerland)"
- press Alt-;, then E to produce é