Created attachment 373147 [details]
Screenshot: client report

Company name and address fields are treated differently from the client's fields. The softare will srash right after the File -> Properties -> Business fields are updated. The similar issue was with rev. 3.3., just no crash after the info was updated. 
The snippet from the invoice (rev 3.3):
<div class="invoice-title">Sąskaita faktūra #000010</div>
...
<span class="invoice-in-progress">Sąskaita faktūra kuriama...</span>
...
<div class="company-table"><table border="0" cellpadding="4" cellspacing="0"><tbody><tr><td><div class="maybe-align-right company-name">Eugenijus JanuÅ?keviĨius</div> */ instead of Eugenijus Januškevičius /*
...
<div class="invoice-notes">DĆā€˛Ä?ā‚¬ā€¯koju uĆā€¦Ä€Ā¾ laikĆā€¦Ä€Ā³ apmokĆā€˛Ä?ā‚¬ā€¯tĆā€˛Ä?ā‚¬Ā¦ sĆā€˛Ä?ā‚¬Ā¦skaitĆā€˛Ä?ā‚¬Ā¦.</div> 
*/ instead of "Dėkoju už laiku apmokėtą sąskaitą." /*
...

environment.local:
[Variables]
LANG=lt
LANGUAGE={LANG}

Test data: 
Company name: Grzegorz Brzęczyszczykiewicz
Company Address: Chrząszczyrzewoszyce, powiat Łękołody


It will not depend on the locate, the same is happening with LANG=en_GB

using LANG=lt *.trace files shows
* 23:11:03  WARN <qof.engine> [qof_print_date_buff()] Error processing time64 0: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_dmy_buff()] Error processing year-month-day 2019-1-28: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_buff()] Error processing time64 1548669600: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_dmy_buff()] Error processing year-month-day 2019-1-28: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_dmy_buff()] Error processing year-month-day 2019-1-28: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_dmy_buff()] Error processing year-month-day 2019-1-1: locale::facet::_S_create_c_locale name not valid
* 23:11:03  WARN <qof.engine> [qof_print_date_dmy_buff()] Error processing year-month-day 2019-1-28: locale::facet::_S_create_c_locale name not valid
* 23:11:07  WARN <qof.engine> [gnc_print_time64()] Error processing time64 1548709867: locale::facet::_S_create_c_locale name not valid

Using LANG=en_GB *.trace files shows
* 00:17:13  CRIT <Gtk> gtk_text_buffer_emit_insert: assertion 'g_utf8_validate (text, len, NULL)' failed