Convert language name to lower case when looking for i18n file

i18n files use lower case names even though variants are usually
written in upper case (e.g. zh-HK -> zh-hk.json)

Change-Id: Iaa646f02b3dd3c61a80c3dd4f38d1cabda2f8478
Merge "Convert language name to lower case when looking for i18n file"