Additionally remember that in an expansionist and/or colonial empire loanwords are likely to sneak in from the territories. Languages also seem to have their own character in how easily they accept loanwords, from Mandarin's [2% loanwords] "native compounds can describe the concept better than your barbarian original" to English's [75%] "give us all your vocabulary in used notes and don't even think about pressing that alarm"+Yes, these characters tend to stem in a large part from identifiable historical, cultural and even grammatical/phonetic concerns. So perhaps you want to decide on a characteristic attitude to borrowing (or more than one, through different time periods), then make up reasons to justify it.
Pro-loanword factors:
- Close prolonged contact with another language
- Existence of a non-native prestige language
- Phonotactic adaptability [? a particularly weak one I think]
- Existing accepted history of loaning
Anti-loanword factors:
- Lack of sustained contact with other languages
- Cultural resistance at a sufficiently grass-roots or enforced level (sorry Académie française, you don't cut it)
- Language is a prestige language in its broad cultural sphere
- Difficult to adapt potential loans to existing phonetic/morphological patterns [weak]