I am experiencing an issue whereby the default (English) text is loaded for a site in Hungarian. The proper translation files are in place.
1) Thought the translation file may have worked if moved to /wp-content/languages, so I moved limit-login-attempts-hu_HU.mo and limit-login-attempts-hu_HU.po to the /wp-content/languages/ directory. This had no effect.
2) On the outside chance it was a permissions issue, I changed the ownership from nobody:nogroup to www-data:www-data. This, also, had no effect.
I suspect the fact that this is a "Must-Use" plugin may be the culprit; since the folder is a bit different. (/wp-content/mu-plugins/ vs /wp-content/plugins/).