Fix: Translation percentage sometimes goes beyond 100

A missing parseInt was causing 0s appended to numbers instead of
adding, causing wrong calculation

Testplan:
Translate
Special:ContentTranslation&page=Before+This+World&from=en&to=vi
Click the section "Background". Without this patch, you will see the
progressbar growing beyond the limits. With this patch, it should
never go beyond 100%

Bug: T98419
Change-Id: I5186c47ee52fea1ecc14bb4e4d522099ec8784d0