ffmpeg copy highest quality audio track of a specific language
A map option will select all streams matched by the specification. The 'best' stream is only selected in the absence of any map options.

So, you'll need a two-stage process to do this:

ffmpeg -i in.mkv -map 0:a:m:language:eng -vn -acodec copy -f nut - | ffmpeg -f nut -i - -c copy output.flac


The first ffmpeg process sends all English language audio streams to the 2nd ffmpeg process, which will select only the 'best' among the received streams for output.

If you want to keep metadata, and not worry about the codec of the input file, you can change your output to .mkv