Is it safe to just remove the other language FAQs? Does English get used as default? I'd assume it's ok, but maybe someone knows that it causes problems?

ie: faq_(insert language).php files

It will produce a "page not found error" if someone uses a languagem, which faq has been removed.
You are safe, if you delete the language entirely, so that it can't be choosen in the users profile.

remove all FAQ without your FQA file from disk and to make english default and olny one enable for forum, you should edit editdisplay.tmpl and:
find
<select name = "chosenlanguage" class="formboxes">
<option value = "english" $english>{$ubbt_lang['L_ENGLISH']}</option>
<option value = "big5" $big5>{$ubbt_lang['L_BIG5']}</option>
<option value = "chinese" $chinese>{$ubbt_lang['L_CHINESE']}</option>
<option value = "danish" $danish>{$ubbt_lang['L_DANISH']}</option>
<option value = "dutch" $dutch>{$ubbt_lang['L_DUTCH']}</option>
<option value = "french" $french>{$ubbt_lang['L_FRENCH']}</option>
<option value = "german" $german>{$ubbt_lang['L_GERMAN']}</option>
<option value = "hungarian" $hungarian>{$ubbt_lang['L_HUNGARIAN']}</option>
<option value = "italian" $italian>{$ubbt_lang['L_ITALIAN']}</option>
<option value = "norwegian" $norwegian>{$ubbt_lang['NORWEGIAN']}</option>
<option value = "polish" $polish>{$ubbt_lang['L_POLISH']}</option>
<option value = "portuguese" $portuguese>{$ubbt_lang['L_PORTUGESE']}</option>
<option value = "romanian" $romanian>{$ubbt_lang['L_ROMANIAN']}</option>
<option value = "russian" $russian>{$ubbt_lang['L_RUSSIAN']}</option>
<option value = "spanish" $spanish>{$ubbt_lang['L_SPANISH']}</option>
<option value = "swedish" $swedish>{$ubbt_lang['L_SWEDISH']}</option>
</select>

change for

<select name = "chosenlanguage" class="formboxes">
<option value = "english" $english>{$ubbt_lang['L_ENGLISH']}</option>
</select>