Multi-word Latin terms etc. used in English

**Describe the bug**
Terms like this that contain one or all words that are not English words in their own right are not currently well supported. This issue is to collect some in one place.

**Expected behavior**
Support these terms like single-word terms for spelling and perhaps grammar checking.

**Lists**

From Latin:
- de facto
- mea culpa
- modus operandi
- per capita
- quid pro quo

From French:
- deja vu / déjà vu
- papier mache / papier mâché

From Chinese:
- kung fu

From English:
- holy moly / holy moley

Start by tracing how Harper handles single-word terms in its spelling and grammar-checking paths, then compare that behavior with the listed Latin, French, Chinese, and English phrases. Done means the listed multi-word terms receive equivalent spelling and, where applicable, grammar checking, with tests demonstrating the expected behavior.

Written by the indexing model from the issue text.