Validate tags in GUI string translations

[fcxSanya]

GUI strings may include tags (for example [font="<...>"]), so the text renderer is trying to parse sequences starting from [ character as tags, what results in errors if translators lose the escape character \ before [ (examples: #3706 and #3831). Currently such issues are likely to stay unnoticed until someone will open a corresponding GUI page using a language with broken translation. It's preferable to perform an automated check before releases, like it's already done for links (see ReleaseProcessDraft: Translation check).