ci: improve readability of sentence_count check

[andreasbrett]

Improve readability of the difference in counted sentences (see #2630). Especially beneficial with larger numbers (compare 100000000 to 100,000,000). Also added relative difference in percent.

Before:

Counting previous sentences for language: en: 403495
Counting **NEW** sentences for language: en: 103495

New count: -300000

After:

Counting previous sentences for language: en: 403495
Counting **NEW** sentences for language: en: 103495

New count: -300,000 (-74%)

[balloob]

I temporarily removed the github action because it was causing a CI failure on some PRs.