The word "mieszkań" (PL) turn-off spell-checking in rest od file #327

Description

While writing the article in Polish (Github markdown, pl-PL), I noticed that in the middle of the document I lost the functionality of spell-checking. Clear typos were not underlined. Wrong words at the beginning of the document were not underlined later. I started to delete the text and it turned out that the occurrence of a Polish word "mieszkań" in the text immediately turns off spell-checking in the further part of the document.

Steps to Reproduce

1. Open Github markdown document
2. Check if spell-check is active and you have installed dictionary for Polish language in system (PL) and added locale pl-PL in spell-check settings
3. Paste text prepared below:

mieszka
Janósz
ń
Janósz
mieszkań
Janósz

Expected behavior:

Each occurrence of the word Janósz, should be underlined, and under the right mouse button there should be a proposal to change on Janusz (Polish name right spelling)

Actual behavior:

The first and second occurrence is underlined. The third occurrence is not underlined. It's after the word "mieszkań"

Reproduces how often:

Always

Versions

Atom    : 1.45.0
Electron: 4.2.7
Chrome  : 69.0.3497.128
Node    : 10.11.0

apm  2.4.3
npm  6.2.0
node 10.2.1 x64
atom 1.45.0
python 3.7.7
git 2.25.4

running on Fedora 31

Screenshot