This is not a regression but a deliberate decision: T213658: Allow translators to distinguish between two single apostrophes '' (markup) vs. " quotation marks in strings to be translated.

The source message is now in the same font as the editor, based on the editfont preference. This is consistent with how MediaWiki treats raw wikitext elsewhere in the interface (such as in diffs, Special:ExpandTemplates, etc). I'd be interested in hearing specifically why this change is bad, otherwise, I'd suggest closing as invalid.

If you absolutely do want a monospace editor and a sans-serif source message, add the following to your common.css.

.mw-special-Translate .sourcemessage.mw-editfont-monospace {
    font-family: inherit;
}