I sometimes write in french my subtitles, and in french we have a lot of diacritical Mark (signe diacritique comme les accents, tréma et cédille).
In rare case, the font does not support the acute accent or other diacritals marks.

How to know if a font support these marks

Check if the font is adding to the sytem

The first thing I do is I write it normally in aegisub.
After I add my font on my OS (see Adding fonts to a Linux OS), I check it with Aegisub (File > Fonts Collector > Check fonts for availability and click on start).

When a font is not available (Could not find font 'xxxx'. Used on lines: xxx), just add it.

In these two examples, I put the font in .font folder, that’s a bad practice!
They must be on .local/share/fonts.

See that the font isn’t available for the glyph

Else if the font can’t show your mark, it’s written Found 'xxx' at '/home/xxx/.fonts/font.ttf' 'xxx' is missing the following glyphs used: ’. Used on lines: xx".
And at the end: One font was found, but was missing glyphs used in the script..

Some glyphes

• For é, write a e and add \frz127.1\ with '
• For è, write a e and add \frz13.67\ with '
• For œ, write {\fsp-5}o{\fsp1}e