Please use plural form format for "Importing %1 items"

For my understanding, can you explain why that would result in grammar errors?

The above way works for English and languages which happen to have a similar pattern to English in terms of plurals. But many if not most languages have other patterns. Some have more plural forms - such as Scottish Gaelic which has 4:
1 or 11
2 or 12
3-10, 13-20
Anything else

For example, for "Importing %1 items" this inflects like this in Gaelic:
1 or 11: Ag ion-phortadh %1 bhall
2 or 12: Ag ion-phortadh %1 bhall (in this specific case, overlaps with 1/11 but not always)
3-10, 13-20: Ag ion-phortadh %1 buill
Anything else: Ag ion-phortadh %1 ball

Others (like many Slavonic languages) have rules about
Ending in 1
Ending in 2
Ending in 3-8
and so on.

CLDR has a good overview over the different types for developers: http://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html

Thanks. Makes sense to me now.