Translation doesn't work

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hello,
We have an guestbook on our website ( default guestbook ). and we would like to change te text on the submit button. so we made an .mo file. afther uploading nothing happend. so we upgrades to 5.5.1 and saw that the dashboard was translated. but the button still isn't translated. i have looked in the code and the t() function is being used. i have also checked my po file and the msgid is correct.

does anyone have an idea?

websignaal