How do I escape braces and apostrophes in ICU message format? 

Wrap a literal brace in ASCII apostrophes, '{' and '}', and write a literal apostrophe as two apostrophes, ''. In ICU's default apostrophe mode a single apostrophe only starts quoting when it immediately precedes a syntax character, so a plain don't in the text is printed as is.

The ASCII apostrophe (U+0027) is the quoting character of the syntax. Everything between it and the next apostrophe is copied literally, which is how you print braces without starting an argument:

Use '{name}' as the placeholder, not {name}.

With name set to Ann this prints Use {name} as the placeholder, not Ann. Two adjacent apostrophes always produce one apostrophe, inside or outside quoted text:

This '{isn''t}' a placeholder.

Output: This {isn't} a placeholder.

Which apostrophes start quoting depends on the implementation:

• ICU4C, ICU4J, Android's android.icu.text.MessageFormat, PHP's MessageFormatter and FormatJS use ICU's "double optional" mode, the default since ICU 4.8. An apostrophe starts quoted text only when it immediately precedes { or }, a # inside a plural branch, or a | inside the old choice format. Every other apostrophe is literal, so It's {count} items prints It's 3 items. FormatJS additionally lets an apostrophe quote < and > because of its rich-text tags.
• java.text.MessageFormat treats every single apostrophe as the start of quoted text, the mode ICU calls "double required". There you must write It''s {0} items; the pattern It's {0} items quotes everything after the apostrophe and prints Its {0} items.

For user-facing text the safest choice is the typographic apostrophe ’ (U+2019). It is never a quoting character in any implementation, it is the correct glyph in English prose, and both the ICU user guide and FormatJS recommend it for human-readable strings, leaving U+0027 to the syntax.

Pitfalls:

• A quote runs to the next apostrophe or to the end of the message, so one stray '{ silently turns every later placeholder into literal text.
• '' is portable, since both modes turn a pair into one apostrophe. A single apostrophe not followed by a syntax character is the only case where ICU and the JDK disagree, which matters when the same resource file is read by both.

The ICU message editor shows the output of a message, which is a quick way to check a quoted pattern.