"The forum is primarily English-based simply because the majority of Fiverr users can all communicate in English making it a simple choice for a common language. It is not against the rules to post in other languages, but users should be aware that non-English posts may end up in editorial review for an indefinite period. This is not due to discrimination or problems with any specific language. We do not have moderators who can read every language, therefore we cannot be rapidly certain that posts are in line with the rules. When there isn’t time to use software translation or the translation is too poor to use, the post will be moved to a pending review queue for further examination. Some will be returned to the forum at a later time. We recommend that users consider posting in English. Another idea is to make your post in English and then repeat it in any other language of your choice, making it clear that it is a translation. Be aware that if a user abuses this idea by post an incorrect translation, the user may be subject to warning, temporary suspension, or permanent ban."
It's not against the rules but it is recommended to use english in order to keep forum running smoothly.