If I search for "dog" on commons here is the query I get

Notice that one of the match queries is on description.en with boost=0.019

If I search for "chó" (the Vietnamese for dog) on commons | here is the query I get

Notice that there is no match query description.vi, because we don't have stemming for Vietnamese. description.vi.plain is present in the query, but its boost is set to zero

This means that search will be less accurate for languages for which we don't have stemming (e.g. Vietnamese, Cebuano, Bengali)

Proposed fix:

• if the boost for a language-aware non-stemming field is zero AND there is no stemmed version of the field, then set its boost to the equivalent value for the stemming field