Use static variable for frequent i18n() call.

Test Plan:
perf stat -r ./bench_parser heaptrack.david.18594.gz

Before: 160M cycles, 259M instructions, 43.58ms
After: 157M cycles, 252M instructions, 42.93ms

Reviewers: mwolff

Reviewed By: mwolff

Differential Revision: https://phabricator.kde.org/D24095