If you install a translation that old with the newest version, you'll get a mix of English and Spanish, not to speak of the descriptions for mechanics have been changed but kept their text format, which will show Spanish text as it was in RC15 but function as changed in RC18.
It's up to you, but personally I wouldn't give up the latest version for an accurate translation. Your English looks pretty good, are you doing a setup for someone else? In that case it might be worth downgrading down to the translation version.
BrolyLSSJ5
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